They could, but they're never gonna if they have access to the characters that are proven to make money. If they have popular characters like Iron Man and Captain America available they'll stick with those rather than introducing lesser known underrated characters. This is a business after all.
True, but a good franchise doesn't require you to watch everything that came before either. If a kid wants to read a Marvel comic today they shouldn't be expected to have read every single issue since 1939. That'd be stupid.
I don't doubt it. Even in this forum some people have been saying since Endgame that they should recast those characters by 2025 because the MCU can't exist without them. But personally I'd like to see a version of the Marvel Universe that's allowed to develop in real time. In the comics there's always this return to the status quo where all popular characters come back from the death, everyone barely ages, and some characters like Spider-Man, Hulk, Wolverine, Iron Man, Captain America, etc. will always come back to the forefront of what's going on. For once I'd like to see a version where people stay dead, do age, different characters do become the main focal point, etc.
For me the longevity of the MCU depends on two things: 1. Are people still coming to see it such that it is sustainable? 2. Is there still someone sensible in charge like Feige, who knows what they're doing and isn't completely destroying the continuity? For me the MCU should continue its linear existence as long as those questions can be answered with 'yes'.