Man, I understand it if people enjoyed Guardians of the Galaxy and think Chris Pratt is a funny dude, but he's not Indiana Jones. Chris Pratt is your funny older brother - Harrison Ford is your sarcastic cool uncle. Pratt can do the funny leading action man, but he isn't the John Wayne-kind of cool that Ford is.
Also, Chris Pratt being the lead in three of the biggest adventure movie series at the same time seems like overkill. There are more actors in the world - being good in one role doesn't mean you should just cast him in everything.
(I also think someone else taking over the role is unnecessary - the world of Indiana Jones isn't this big place that needs to be explored and elaborated upon like Star Wars. It's all about the leading man - take Harrison Ford out of the equation and it's just one of those dozens of Indy-franchise clones that came out in the 90s. The only thing that would be recognizable as Indiana Jones would be the hat, the theme and the title. Just make something new then. Indiana Jones was my favorite thing in the world as a kid, but it's not something that needs a thousand sequels. I'm up for one last (hopefully decent) Indy with Ford in the lead, but if we can't have Ford, I'm fine with letting the franchise be. That's not going to happen obviously - Disney would want to get at least something out of that license)
Edit: If I had to cast someone new in the role (and I hope they won't, but just for the sake of argument), I'd want them to go for a relatively unknown actor. Or here's another (stupid) idea:
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