Alien would have been more like
Alien 2: On Earth without the budget. Or possibly close to
Planet of the Vampires (the benchmark for sci fi horror at the time).
Predator could have been another cheesy 80s action flick like
Missing in Action with Chuck Norris (or many of the other movies trying to rip off Rambo: First Blood). Also see
Without Warning, which is a low budget 80s monster movie with an alien hunting humans (alien being played by Kevin Peter Hall, no less).
(Not bashing any of these movies, btw)
It wasn't just budget, though, but the talent behind it. Anyway, enough of my rambling.