While I was looking forward to a new actor playing Doom, I can't say I totally disapprove of Downey being cast. It could be interesting.
As far as the debate on re-casting a popular actor from a popular franchise, I think Marvel/Disney is just realizing that the success of the MCU has been a combination of specific actors combined with popular characters. Iron Man succeeded directly because of Downey, and it was a character that a lot of people were excited to finally see onscreen. Captain America is still Chris Evans for a lot of people, so it remains to be seen whether Anthony Mackie can carry the franchise himself (one could argue that The Falcon & Winter Soldier wasn't that successful, and Sebatian Stan was shouldering half the load, plus it was a streaming series vs a big budget theatrical release). I'm guessing the new Captain America film will still do well, but fall short of other Marvel films, including Deadpool & Wolverine (which had a huge opening already, but we'll see if it holds - maybe all the fans came out early to avoid spoilers like I did).
Looking at other franchises that keep trying to recapture the audiences they had, you can see how impotant the original actors are to the franchise success. The Aliens movies (with another installment coming soon) suffer (in my opinion) when not having the character of Ripley in them, and Sigorney Weaver is Ripley. But even changing the character & story, like they did in Alien³, where she played a clone of Ripley, messed with the established character too much and didn't resonate with the audience.
The Terminator franchise similarly struggles without both Schwarzenegger and Linda Hamilton. I enjoyed Terminator: Dark Fate, which reunited the two, but they still altered the characters in a way that wasn't totally satisfying for many fans. The first three films followed largely the same formula, but the third didn't have Hamilton come back as Sarah Connor. It also recast John Connor, and though I doubt keeping Edward Furlong would've been a big draw, it still recast a main character from the prior film. Terminator Salvation and Genisys also failed to recapture the audience, again without the leads - Salvation just having a digital Schwarzenegger cameo, and Genisys recasting Sarah and John Connor, and completely messing with the story and timeline (as I said with Weaver as Ripley, Hamilton is Sarah Connor). In fact, Genisys also spolied the reveal of John Connor in their trailers, which seemed like a mistake to me.
The Star Wars films arguably suffer from similar problems, especially the sequel trilogy and various other films and streaming series. Trying to recapture the "magic" of the original cast, characters and story of a franchise isn't easy.
So it remains to be seen if Downey is a big enough draw without also playing Tony Stark. History suggests it may not work as expected, but Disney is at least realizing that the actors are a big reason for the success of the MCU.
