That's my reasoning though, it worked before because as you say Thor and Cap were wildcards. Avengers 2 however IS a surefire hit, and other than that I think if the actors get shortshafted the VFX dudes must have it really bad. Although I was wrong on the Whedon assumption.
Should their contracts should be up (RDJ's is) then Marvel should do their best to get the leading actors to return. ScarJo and Renner are expendable. But if the actors signed a contract, then they should honor it and find better agents if they want more money the next time around. ScarJo will have been in Iron Man 2, Avengers, and Capt America 2. If that's all she manages to get in, then that is fine by me so I can see more Marvel heroes making their debuts.