They should have stopped at The Last Crusade but then we got the monstrosity Kingdom Skull so it makes sense to make an attempt to end the franchise on a higher note.
I'm sure Harrison Ford has a clause in his contract that states no new Indiana Jones reboots while he is alive. It's the end of the line for Harrison Ford as Indy but Disney won't shelf this IP for long. The dial on this franchise is ticking for a future installment I'm sure. Minus Harrison Ford.