You think Avengers or the MCU will ever go an X-Men Last Stand route and kill off a bunch of heroes? I'm talking way down the line, of course.
I doubt it. They want this to be a long-term thing. If and when they kill off heroes, they're going to think about it
really hard ahead of time, and I doubt they'll ever kill off a bunch at once.
X-Men Last Stand did it for shock value (or in the case of Cyclops, contractual bullshit) and didn't really consider the future implications of anything that they did. They depowered and killed so many characters that there wasn't really anywhere else to take the series, hence why they've done nothing but prequels since and will probably go for a full reboot before they do an actual X-Men 4.
If and when Marvel starts to kill off heroes, I hope they never fall into the crap comics have and start bringing characters back to life or retconning deaths, thus ruining any impact those character deaths would have. Instead of, "Oh shit he died!" it's, "Well is he
really dead?" or, "Gee, I wonder how long until they bring him back to life."
Incidentally, X-Men Last Stand managed to also make the mistake of stupid retcons and ruining the concept of character death in the same movie, when Magneto seems to be getting his powers back at the end and Xavier somehow lives on in the post-credits scene.