Joke’s on you. In the Marvel fandom, a multiverse of endless possibilities exists. Therefore, the “official canon” I choose to accept can pretty much be anything I want it to be. If characters I like die in a movie, I simply ignore it at this point, knowing there are (canonically) infinite versions of those characters that DIDN’T die, and even in the main timeline, multiple characters “die” over and over again and still come back to life.
This is all ignoring how many Marvel characters wouldn’t even be killed simply by “gunning them down”, because their powers make them indestructible or whatever. Good luck gunning down Wolverine or Deadpool in a bloody massacre. Best you’ll do is make them mad.
This is also ignoring that there are multiple versions of the Marvel characters that exist outside the MCU. There’s still all the various comic book versions of the characters, animated TV series, etc. So what if the characters were to all die in the last installment of the MCU, it won’t stop people from still being fans of those characters and consuming other media that involves them.
No actual Marvel fan would be bothered by this. Some might even enjoy it.