The failure, I think, comes from the studio over-reaching, expecting bigger amounts of money from a niche, horror-based franchise that cannot compete against other franchises in the current market. There's nothing intrinsically bad about Prometheus or Covenant, most people liked them (don't believe the personally offended people on the internet that bag these movies), but this franchise just doesn't pull the numbers required that would make them hugely "successful," i.e. wading in cash.
The latest 'Iron Man Pred Tech suit' thingy from The Predator really shows a desperate attempt by Fox to appeal to wider audience, but I think this is the wrong way of doing it.
I think they really have no choice but to produce lower budgeted and more intimate movies, as difficult as that might be, and then profit from that. Another avenue that would benefit them would be a Netflix series of some sort, showing side-quels, or even if it was to have a show based on something from the EU.