I really think this is probably the last. It took Lucas forever to come up with another good idea for a film and then it took over a decade to make that actually happen. Ford is already 65 and by the time another gets made he'd probably be 67 or so. Not saying that would really make much of a difference, but it might.
PLUS. We've explored most aspects of his character by now. The only thing left really is to see how his age has affected him. On knowledge/wisdom, maturity, physicality, financially and possibly even in love. I don't know if it's been confirmed or not since I've been staying away lately... but if Mutt is indeed his son that's just another aspect of his character we'd explore that would be exhausted after KOTCS.
Another thing, the year in this film is 1957, and it is likely a future film would be set in or around the same time-frame and wouldn't (or at least shouldn't) center around Russians again.
Finally, I think they'd be out of good ideas for stuff like the "phobia" stuff and such. Like the snakes, rats, bugs, etc.
And you know what. I really really don't want to see the Adventures of Mutt. I like Shia, but this is not his series. I don't care about him. Even if I like his character I don't want to see movies about him adventuring. I only care about Dr. Indiana Jones thank you very much. His name is in the title, this is his adventures and his stories. So I don't really want or care to see the franchise carry on in spin-off sequels.
There is an expression that goes, I think, "One more brush stroke would ruin the painting." Sometimes, things are good as they are. You think you need more but more would be overkill.
Also, "All good things must come to an end." Everything. Life itself does not last forever, neither does the Indiana Jones series.
ALLLLLlllllll that being said. Doesn't mean I don't kind of want to see another. But I haven't even seen this one yet!? If they could come up with something worth seeing, TRULY worth seeing, I'd probably still go see it.