The ability to buy better weapons ruins the game twice over. Either you're paying real money to beat a game you already bought, which is how your self-esteem wins a pyrrhic victory, or you're struggling through battles with the knowledge you're deliberately wasting time that even the game wants you to skip. Hard levels are no longer finely-balanced tests, but blatant difficulty spikes added as aggressive sales pitches. This shatters your immersion, keeping you constantly aware that you're not escaping from reality by struggling against alien monsters; you're losing a real battle against an evil Terran corporation.
Solution: No Micropayments Mode
Every game should have a simple "I WILL NEVER BUY ANYTHING" toggle which removes all offers and options from every menu. Not greying them out, not defaulting them to "No" -- absolutely eliminating even the least glimpse of them from the game. If only to reveal how many holes are left. This would have to be instituted as a legal requirement, just like age ratings, and it could be forced for the same reasons. Parents used to be worried about violent games, but now they face the real problem of children being tricked out of thousands of dollars. Micropayments are a genuine scam that exploit children as a security flaw. Video games are meant to make dealing with kids easier, not worse.