I always find it kind of weird that some characters get so tied up in specific actors. For example, I saw a headline that Hank Pym of Marvel, played by Michael Douglas, will be retired out of the movies because Mr. Douglas won't play him any more.
I mean, my first reaction is that your perplexity comes from trusting a headline to be a largely accurate summation of the situation.
In the MCU they've recast roles before - we've had three different Bruce Banners, two James Rhodes, two Thaddeus Rosses, two Billy Maximoffs, three Howard Starks, three Cassie Langs if I recall correctly, over a dozen voice-recasts in
What If...?, and so on. Reaching beyond that we've had several Fantastic Fours, Spider-Men, a multitude of X-men getting recasts....
So, really, I think we can put to bed the idea that there's some reluctance to recast roles, in general.
In most cases (like Hank Pym) it really doesn't matter.
In a few cases, individual actors become so iconic and beloved for their roles that recasting invites a lot of direct comparison that's apt to be negative, which is bad for business.
There have been a couple times where it has been done well, and even used to tell interesting stories - Professor Xavier and Magneto, for example, and Spider-Man have been recast to good effect.
And that's the key there. For someone who has thoroughly
owned their role - like Robert Downey Jr. has owned Tony Stark, there's a risk that the next actor will fall in the comparison, so you don't risk it
just because. You risk it if, and only if, you have some awesome story that will be enabled in that recasting.
Maybe it's because I come from such a theater background, but, I've never really tied a character with a specific actor.
The effects of recasting are still apparent in theater, though. Pretty much every hit show on Broadway starts with some casting, and the actors define those roles, and then go on to do other things. And the next person in the role has to deal with the precedent - we see this in shows like Les Mis or Phantom of the Opera, or Hamiton...
You even see it in movie/theater crossover. Robin Williams
owned the Genie in the animated Aladdin. They
don't even try to replicate that in the stage production - they take the Genie in a different direction, to allow the actor there some space.