If you’ve seen the test footage of Selleck in the role (playing the scene in the Raven when he’s first reunited with Marion), he is actually really good. Different, but good, and highly credible. It’s easy to see why he was initially cast.
But there’s no way the character would’ve been the same. Each actor brings his own kind of energy, so it’s hard to imagine anyone else but Ford doing it now, but Spielberg was directing and it’s not like he had a shorthand with Ford (they’d never worked together previously) so the quality of the adventure would’ve been comparable but the tone of the film would’ve been altered, in likely a significant way.
Also, physically speaking, Tom is a big guy–not just taller, but in terms of mass. It might’ve been a little harder seeing him as a professor who moonlights as hero and museum nerd.