TheBoost said:

Cooper and Pratt are both too funny.

Harrison Ford looked, and sounded like a stoic action hero. So whenever Indiana fumbled, or was goofy, it seemed almost out of place. I think therein lied the charm of the character.

You nailed it.
I'd never believe in Chris Pratt as Dr. Jones because I know his real-life personality is nothing like him. Harrison, on the other hand, seemed like he really could have been that guy.

But, that's so rarely the case. usually an actor (especially a "known" actor) really has to work hard to make the audience stop thinking of them as their real-life selves and create a very different character that the audience can believe in..
Whoever tries to follow Harrisons performance would also have the added grief of trying to make the audience accept them as a character we're already so familiar with..
I wouldn't envy any actor in that situation...