Things can just be fun, it does not have to have rules.....Even though the multiverse does have rules, and was established for years in the MCU.
There is quite a bit of push/pull here. Can this all work out the way the MCU and The Big Mouse hopes? Yes, it's possible. RDJ in general has been fantastic in the MCU, he's charismatic and he offers a type of "cost certainty" as a known quantity from the production side. There's comfort in that logistically.
However stunt casting RDJ will begin to corner the writers room. The only Tony Stark "thread" left from Phase 1-3 with narrative juice is the storyline about Tony assessing if he could out technology /out build a potential future mass attack like the Chitauri swarming New York. Also the "Multi-Verse" plotlines have all turned into a common trope now ( Main character tries to change the past or change history, then realizes it all blows up in their face, then comes to terms that they can only change themselves, they can't truly change the world around them. If you take away the future, you take away hope) And how many times can you beat that "Multi-Verse Is A Ticking Time Bomb" horse to death? The Tony Stark character development was built, in part, off of a composite including real life Elon Musk. How do you paint more Stark as a pure genius without uplifting the Musk elements within it? The Big Mouse is going to want to tightrope in taking huge pot shots at Musk via proxy through a Stark/Doom character. Now you've subverted the writers room beyond just telling a good story.
The push here is a narrower scope within the writing, where you are literally forced into certain storylines and arcs, that might be beneficial for the MCU, which has become far too complicated and bloated with current releases. While the opportunity to kill off a lot of extraneous characters in the MCU has logistical value, it will start to buckle the narrative ( Death for established characters needs to be purposeful. Whistler dying in the first Blade film was purposeful. You needed to push the Blade character beyond his limits to show a full arc. However Whistler dying in Blade Trinity was done to just write the reluctant actor out as fast as possible with no real true impact on any future Blade character development at all. It was pointless and pathetic. A full out all out massacre just to clean house has value, but it has tradeoffs too)
The pull here is once you keep going over covered ground, you are no longer telling a compelling story, you are just making content. You are now an energy drink company, not acting as real filmmakers. The risk here is forcing some Stark like entity back over and over again over preexisting coverage. Also, as mentioned before, it risks taking Tom Holland/Peter Parker, and actually setting that entire storyline/narrative in a backwards direction. Can you patch over old ground for two major films like two new Avengers films? That's taking a single gallon of paint and hoping to redo all of Yankee Stadium with it.
Yes, it could work out. RDJ is a lot of fun. He understands how to inject real charisma with the role and create a lot of interest from legacy fans. But from a functional writing standpoint, this is asking for some real heavy lifting. There are clear logistical and narrative pitfalls and road blocks that are just of unavoidable at this point. And if the writing falls apart, the films will fall apart too.
Yes, I agree with you that the "Multi-Verse" begins to shed some basic rules. That creates different openings and opportunities. I won't deny that.
But if you want functional writing and you want to give your writers room a fighting chance, you do need practical boundaries. You simply can't find this "fun" without some kind of structure. What you are saying is true, but you've stripped it of all reasonable context to get there.
As a fan, I hope it works out. If not, as a pure collector, I hope we all get some damn good toys of it. Barring that, I hope at least fans get some spectacle like watching Simu Liu getting burned alive. But it's still stunt casting and it's a huge gamble.