I figured Marvel would just shell out for minor cameos like Tony Stark in Spider-Man or whatever for a couple years before "rebooting" the characters down the line. Endgame has a bunch of people on one-film contracts.
Plus none of the characters they killed are the ones they have to theoretically replace. Stark, Cap, Black Widow, Thor, etc. are all the main characters in Endgame. It was people like Gamora or T'Challa or Drax or Strange, who are already signed for sequels in their films, who were vaporized.