M. you might be right when you say that RDJ cannot play Tony Stark forever in case Marvel wants/would like to transform the Avengers into movies "à la James Bond".
But coming here and say that RDJ is full of himself, i think you should expect these sort of reactions.
This said and in the hypothesis of the above happening, i'm pretty sure RDJ's Tony Stark will be considered the best one. (As i'm considering Sean Connery the best and unique James Bond, even though i enjoy almost all the JB movies). Ok, as of now we don't have terms to make comparisons, but it will be quite difficult to portrait a better Tony Stark than the actual one. And this is not said only from "us", RDJ fans, but also from comic book geeks who consider RDJ the personification of Tony Stark. I have read Avengers reviews of pre-screenings for fans with various comments for all characters/actors and when it comes to RDJ it is usual to find the words "oh well, what can i say about RDJ, he is Tony Stark, so..."
Aaaaaanyway...time will tell....and we all be here to discuss again