It's more than a cameo, I agree, but he's not THE lead, as he would be expected to be in an Indy movie. He's part of an ensemble. Plus, Han was never as physically active as Indy. He spent most of ESB sitting down in the Falcon's cockpit... there's no reason at all a 72 year old can't do that. Indy, on the other hand, should be leaping from horses, dragged behind trucks, swinging on ropes/whips, getting into fistfights, etc, etc. And a 72 year old simply can't/shouldn't do those things. Sure, the film makers could do it with special effects (CGI, head-replacement, etc), just like they had Christopher Lee fighting in the SW prequels, but do we really want that? I don't, and I don't think many other people do either. Nor do I think many people want an Indy movie that lacks action simply because the leading man is too old to partake in the action. So, regrettably, that means the only way to make another good Indy movie is to find a new leading man.
Anyway, aside from Harrison's age, there's also the time period to consider. If Harrison is cast, the movie would have to take place in the 1960s, at least... more likely the 1970s. Does that time period really work for an Indy movie? For me, it wouldn't. It's too modern, and it would lose some of the charm that we got from the 1930s setting.