In the mid 80's Spielberg's positivism hadn't fully been realized. Post Empire of the Sun anything is possible. I don't think he ever had that much influence on SW, but if he had directed ROTJ then things would be completely different. (More ewoks for example.) I do think he made Raiders brilliantly because he knew everything was riding on it for his career to continue. Temple is a simpler serial film that happened to be made in 1984. Last Crusade is a tired and lumbering dinosaur.
As for Indy IV, I'd blame bad scripts, bad choices, both Lucas and Spielberg, David Koepp, and the stupid decision to wait so damn long.
The real question is: Who the hell is lobotomizing our filmmakers?