Not saying he would have been bad or anything, but... can you really picture that? I can't. For me, Ford defines Indiana Jones as a character... his strength, his courage, his smart-mouth, his unexpected tenderness... and of course, his zeal for the hunt and his tenacity never to give up. You can tell that Indy's character has made a big impact on Harrison himself: in the bonus DVD features, Ford speaks of Indy's quests as his own ("...I have to rescue my dad..."); he has become tied with the character and has given Indy Jones yet another demension for audiences to appreciate.
Another thing: seeing as Ford has just finished the 4TH Indy film, and Tom Selleck hasn't done any, don't you think this is kind of a moot point?