And these movies are pretty good at wrecking stuff.
Think about it. When Die Hard came out, with a hero who could actually bleed and made mistakes, all of the '80s indestructible heroes who wandered through hailstorms of bullets without blinking seemed ridiculous. It's suddenly impossible to relate to Commando, because Arnold approaches a firefight the way you approach a hungover Sunday morning supermarket trip. Or look at movies like Happy Gilmore and Ace Ventura: Pet Detective. Those were raunchy, edgy comedies when they came out, but they've been replaced by raunchier, edgier ones -- not because comedians have gotten edgier, but because the edge has moved. Adam Sandler's schtick has been ruined by crap like Jack & Jill, and as we just pointed out, jokes at the expense of transgender people have moved from "taboo and exciting" to "stupid and offensive."
And finally, show Indiana Jones or Aliens or any of your favorite action movies to a kid who's grown up watching The Avengers, and he or she is going to be bored. (Trust me, I've tried it. If you don't get them before a certain age, it's hopeless). Because children are stupid, and because the rules have changed, what's cool has changed, what's exciting has changed. There are different cues for them to pick up on. And that is exactly what Ultron represents.
Let's start on a superficial level. Like I said at the beginning, Ultron has spread his consciousness across every single robot in his army, which means he can only "die" whenever every single soldier in his army is destroyed. He even shouts "what doesn't kill me makes me stronger" while building a bigger, tougher version of himself, which then shreds the older, weaker, shittier Ultron into pipe cleaner. Just to make sure you got his point. Then, in the climactic battle on a flying city (these movies have gotten crazy weird, man), the characters say over and over that every single Ultron robot has to be destroyed, or else he will live on. It's a pretty cool way to raise the fight's stakes, and it's exactly how the Marvel Cinematic Universe works.
The robots are Marvel movies, and the Avengers are the audience.
Ultron is the MCU, and each movie is one of his robots. Remember The Incredible Hulk? Ha! Of course you don't, because everyone who isn't me hated that movie. And if it had been a standalone story, disconnected from any cinematic universe, we would've had to have a reboot (like with Spider-Man or the Fantastic Four) before we got any more Hulk movies. But because it's part of the MCU, that character got to piggyback on the success of Iron Man, Captain America, and Thor, and show up in The Avengers with no required backstory -- just vague hints that he's "the big guy." At this point, The Avengers was basically the coolest guy in the room, grinning and winking and saying "Remember Hulk? He's cool in movies, right?" And despite the fact that no cool Hulk movies exist in this Earth, we were all "Yeah, man! I love that Hulk guy! His movies are just grand!" because we wanted The Avengers to like us.