That has more to do with the timeline of the original movies, not the characters or the universes being incompatible. Which is what I was talking of. Absurd comparison. Predators have been hunting centuries before the events of the first movie and absolutely nothing hints that they will stop in the future. Which, surprise, is when the Aliens movies take place. I see what you mean, but I still think that it's not impossible to make a good movie with them together. Sure, it probably would be much more of a "simple" action movie, but that's not an inherently bad thing.