The problem with Chris Pratt, as I see it, is that he's a comedian first and you always get this "Andy from Parks and Rec" feel from him that he's adorably stupid. A lovable oaf. I couldn't really take him seriously in Jurassic World partially for that reason. Honestly, when I saw Mad Max earlier this year, I thought that Tom Hardy would be a better Indy, which in a way I simultaneously see as sacrilege since Indy is such an American hero. But Hardy does nevertheless have a gravitas to him that I find lacking in the majority of American actors, and certainly Pratt.
Basically, I think of that awesome scene in the beginning of Raiders at the college with the G-men, and Indy says "The Nazis have found Tanis," and he and Marcus both have this light in their eyes as he says it--that implication that they're reveling in the mystery, like giddy schoolboys for a moment. I can't see Chris Pratt pulling that off. I'd sooner see Bradley Cooper, Chris Pine, or Tom Hardy attempting it though none of them are amazing choices themselves.