Lucas insisted on his stupid aliens idea as a precondition to making the movie, so maybe Spielberg said "screw it", and decided to make an all-out cartoon in response. That damn flying saucer at the end of
KotCS might have been even
more aggravating had the movie living up to that been better than it was.
Unlike Lucas, Spielberg has actually directed more than zero good movies since 1977, including his most recent
Bridge of Spies; so, without Lucas' involvement and lame-ass story demands, it's entirely possible he'll make a sequel worthy of
Raiders. Not
guaranteed, no, but possible.