I think it's silly that people are assuming this is a panicked pivot away from Kang. Doom always played a massive role in Secret Wars, as a surprise villain to take over from the main villain (The Beyonder). I was always assuming Kang was going to be a red herring villain, with Doom stepping forward for Secret Wars.
So with the Kang change, I'm guessing they're simply removing the first step of hanging it all on Kang? But there's still a lot we don't know and I think one major missing piece of that puzzle will be the F4. Both Kang and Doom have obvious lore ties to Reed Richards and the F4, and with the 60's setting (in a different universe too) there's obviously a very specific plan in place here, which makes this approach seem more like rebranding rather than a desperate pivot.
I also wouldn't be surprised if this was something in the works anyway. For the years since Endgame, we've seen so much discourse about whether Iron Man would ever come back, whether RDJ ever came back, whether the Russo's would ever come back. The Russos stance was that they would MAYBE come back for a Secret Wars movie. And they have also been VERY against brining back Tony Stark. So for them to be suddenly championing RDJ as this different iconic role, I would be surprised if it was a total backflip rather than a creative idea that genuinely excites them?
I also think this idea of a Stark variant being a Doctor Doom variant that becomes the God Emperor of Secret Wars is more exciting when you think about the ripples that would send through the remaining characters. I think we can definitely assume that Secret Wars will "Reset" the MCU in a way (Most likely mash together most of the disparate Marvel universes e.g. F4, X-Men etc) into one united universe, tidy up the canon (and maybe even recast any characters that it could make sense for). So the idea of bringing back the man who began this universe and having him play a figure that tries to destroy it, is kind of the perfect synergistic theme to bring the Multiverse Saga to an end (thankfully.)
Anyway, I think seeing the sheer number of "WTF" reactions from people, I'm also willing to guess this was part of the plan. Shock everyone, get people talking and exciting about the MCU again. Bring people back to cinemas just to see what the hell is going on. And from there, I would be very surprised if there aren't more shocks planned.