The only reasons The Last Crusade “must” be a trilogy capper are
the title is Last (but the following word is Crusade and not Adventure or Movie or whatever so it doesn’t even matter)
the final shot is a literal ride into sunset
Just because KOTCS ain’t great doesn’t mean Indy’s story is inherently a 3-part one.
Of course everyone knows that the Star Wars series has reached a clear end with The Last Jedi.