This time 6 years ago maybe they would have. But the idea that guys like RJD and Chris Evans were irreplaceable as Tony Stark and Steve Rogers is now commonly held. Same with Boseman.
But now Marvel Studios is into its replacement hero phase. Much like the Comics of the 90's.
There is a new Capt. America that isn't Steve Rodgers (played by C. Evans), a new Black Widow that isn't Nat Romanoff ( played by ScarJo) , and soon a new Thor and IronHeart Tony Stark's replacement is getting a TV show.
Now that replacing heroes is an option for the MCU ( and it took a while to get the MCU to this point where you can see in story the surviving characters in it wanting to take a break) they can use this option they didn't really have before to try to make the best of a really awful situation.