Yes, but you're a fanboy. You're on a message sight discussing what ifs of a movie franchise's future that are hypothetical.
We're far from the "general public" in terms of these movies and I still firmly believe the reason they see things like Cap and Thor, if they see them(alot of my friends only saw IM and Avengers), is because they play into Avengers and ultimately Iron Man. Its also why RDJ is making 50 million from avengers and the next closest was what like 14 for Sam? Marvel knows this as well.
Simply being an animated pixar type film doesn't cut it anymore, plenty of those are flopping everyday, even from the likes of people who's name should sell like Tim Burton. If they want to sell people on Guardians and talking space animals, it's going to be from hype of it going twoards another Avengers and if people don't have much to look forward to carrying into that, such as a subpar second rate actor like Matt Dillion doing a RDJ impression than it'll turn people off. It's not like they can reboot Iron Man mid Phase 2, whoever takes over has to play the character the same way RDJ has this whole time with the same motives and history. Theres no wiggle room to make Tony Stark your own if you're the actor taking over. Honestly I can't imagine any A-list actor wanting the role in such a wake or expectations and backlash. It'd be virtual career suicide if they blew it.