You have to think that one reason people are so pessimistic these days is that when we were growing up, every third movie involved an apocalypse ... via zombies, robots, monkeys or natural disaster. It can be kind of a downer, but if you want to make yourself feel better, all you need to do is remember that almost none of the apocalyptic scenarios actually work.
What do we mean? Well ...
#5. In the Terminator Series, the Machines Have No One to Build Their Army
In the Terminator series, the American government ill-advisedly relinquishes control of their nuclear defense system to an unfeeling artificial intelligence programmed only to kill. It proceeds to do so indiscriminately, and mankind is reduced to a few scattered rebels fending off hordes of Austrian robots.
"Sir, may I ask why it's giggling?"
So, What's the Problem?
Who or what is building all those Schwarzeneggers?
At the time when Skynet initially nukes the world, it's still just a computer program drifting around the Internet. Sure, it has access to nuclear launch codes, but not to more delicate equipment like opposable thumbs. Even if we assume that, in the world of the film, Skynet is able to hack into and usurp control of every factory and power plant through the Internet, it's still inheriting a world of machines designed to be operated by humans. That means valves need to be turned, buttons need to be pressed and levers need to be pulled. And just in case you're assuming that Terminator takes place in some kind of post-valve world, these scenes from Terminator Salvation would seem to suggest otherwise:
"Wait, I'm not here to kill you. I just need you to show me how these valves work."
"We've got to get away from these valves. If they find us, they'll make us turn them."
If you're thinking that Skynet probably just employs a division of Terminators to operate its factories, you're missing the point -- the computer didn't have access to Terminators back when it killed off all the humans. In fact, the most sophisticated robot that Skynet had access to at the time, the T-1, was a goofy-looking tank with Gatling guns instead of hands that could barely fit through a doorway, let alone turn a valve.
It can play the guitar surprisingly well, though.
It doesn't really matter how smart the computer is -- it can't recommission a car factory, for example, to start churning out Terminators. And even if it could, how is it going to mine the ore, refine the metal and drive it in trucks to those factories? You know, all that stuff that still has to be done by human beings with hands?
In reality, after it nuked everybody, Skynet would probably be left with very little to do in the few days it had to live before something important broke somewhere.
Turns out that beating it was as simple as running Skyrim on its primitive graphics card.
#4. The Demographics in The Matrix Can't Possibly Match the Real World
In the Matrix universe, everyone in the world thinks that they're living perfectly normal lives within a normal (if slightly green-tinted) modern world, when in actuality they're living in a computer simulation and their unconscious bodies are really lying in tubs of pink goop. And, most importantly, they're living in a year that is decades later than anyone realizes.
If the machines are behind all popular culture, The Matrix is a surprisingly optimistic view of humanity.
So, What's the Problem?
There are two important things to know here. First, it's established that everyone in the Matrix looks pretty much the same way they do in the real world, sans body hair and a tan (they go out of their way to point this out, calling it their "residual self-image"). If you're a man in the Matrix, you're a man in the real-world pod. Black people are black, white people are white and Asian people are Asian. And Neo looks like the offspring of his parents, which means that when a baby is born in the Matrix, the genes were taken from parents in the real world.
Now, we know what you're thinking: "That means that every time a man has sex in the Matrix, inside his real-world pod a robot hand comes out and jerks him off and collects the semen. No wonder they hate us!" This is no doubt true, but it's not the point we're making.
It also means there's a flying house-call dildo with AI.
The problem is that the demographics of the Matrix can't match up with the real world.
Remember, we know that the Matrix simulation has been running for at least 50 years (in Reloaded, we meet an old man in the real world who said he was freed from the Matrix when he was 11). That means that when it started, the simulation was pretending it was the year 1950, when the real world was around the year 2150. But the demographics and population of 1950 were hugely different from what they are even now (with Africa's population increasing at a much faster rate than, say, Europe's or Japan's), let alone what they'd look like 200 years into the future.
In fact, the numbers of European and Japanese children in the world are actually shrinking over time. Europe's population of children aged 0 to 14 has shrunk from 143 million in 1950 to 127 million in 2000, and Japan's has gone from 30 million in 1950 to about 18.5 million in 2000. In other words, the machines would be short millions of children and would be forced to take tens of millions of children from Africa and reassign them to be children in Europe and Japan and try to convince the population that they are their naturally born kin.
"Honey, we need to have a talk. Now, I'm not accusing you of cheating or anything ..."
Considering that the Matrix simulation is so fragile that the vaguest idea that something is wrong with the world can break people out of the spell, this is going to be more than just a minor problem for the machines. At the very least, a lot of white women are going to have a lot of explaining to do.
#3. The Monsters in I Am Legend Would Starve Within Weeks
I Am Legend is set 1,000 days after a virus has destroyed humanity. The seemingly lone survivor, the Fresh Prince of Bel Air, lives in Manhattan and gathers food by day while fighting vampirelike plague victims called Darkseekers by night, in search for a cure to turn them back into regular, non-flesh-eating people.
And maybe come up with a good skin cream, because damn.
So, What's the Problem?
After the violence died down, there shouldn't be that many Darkseekers left in Manhattan. Only around, say, 10. That doesn't leave many for Will Smith's experiments.
There are 1.6 million people in Manhattan, and the movie tells us that around 1 percent survived with immunity, while 9 percent became Darkseekers. So, what do these 144,000 monsters eat? It's safe to say that there's not a lot of goods being shipped in from the mainland anymore.
And definitely not with the road department taking every weekend off.
Obviously, the first thing they did was chow down on the roughly 16,000 people who survived without monsterfication. Now, the CDC says the average American weighs 176.5 pounds, so we have about 2.8 million pounds of meat. Assuming human meat has the same calories as pork (about 1,100 calories per pound), each Darkseeker needs to eat about 2 pounds of human per day. So, by feasting on surviving humans, the 144,000 Darkseekers would ensure their survival for a whopping 10 days before everybody starts going hungry.
OK, how about deer? We know they eat deer. At their most densely populated, 60 deer can live in a square-mile area. Manhattan has about four square miles of parkland, so, being generous, there's 240 deer in Manhattan. So, if the Darkseekers eat half of the herd and leave the other half to repopulate, we can get about 21,000 pounds of meat a year from the deer population (about 58 pounds per day), which is enough food for about 19 Darkseekers to survive. Also, there are lions on the island. The adult lions by themselves would eat 27 pounds of deer per day. All told, there's enough food on the island for 10 Darkseekers.
"Simba, it's time I taught you about the food chain -- via a cheerful, bouncy rap tune."
But this is all splitting hairs anyway, considering that Will Smith would have killed them all for his experiments. The cure that he develops is referred to as his 391st attempt, so considering he's most likely wiped out the species he was trying to save, that kind of makes him the bad guy in this situation.