Right, but the time between Chris Evans being Johnny Storm and Steve Rogers was only four years. Many people balked at the casting because he had just portrayed another Marvel hero so recently. Worked out well for him.Human Torch was part of a largely forgotten Fantastic Four from almost two decades ago… not THE icon of Superhero movies for the last 15 years like Downey was with Iron Man.
If this is just RDJ wanting to remain in the MCU by playing a different character, then it seems like very odd casting. His role/arc/etc. was very memorable and for him to just take over another iconic role so soon (5 years since his last Iron Man appearance) shouldn't go over well with fans. How many other actors out there could pull off the role of Victor Von Doom and they just hand it over to RDJ? Meh.
I am just glad it isn't another Ultron-like situation where Doom is an Iron Man offshoot somehow. Hopefully they know what they are doing, but who knows anymore with Disney...