I don't remember them talking about a bunch of characters will die in "AOU".
Regardless, that was never an option. All the actors still had contracts for like 3-4 more movies after "AOU".
Ironman, Cap, Thor, Black Widow, Hulk, Loki and Hawkeye don't have contracts after the 4th Avengers movie. I could be wrong but I don't think Nick Fury does either, counting "Captain Marvel".
That's basically the entirety of The Avengers team. The old one, anyway.
Then there's a gang of side characters that don't have contracts going forward, either. The Collector, most/all of the Nova Corp, etc. will most likely die just based on their proximity to Infinity Stones.
I doubt they'll kill both Ironman and Cap in the same movie but a gang of people will die just based on the fact that their contracts are up.
Fred.