Yeah,just about every horror movie has been devalued in some way or another,by being given a handful of worthless sequels (Hellraiser,Halloween,Child's Play).
The Fly (1986) should have been left as a stand alone movie because it's 1989 sequel was an insult.
In retrospect,they should have left the original Rocky alone.Even though most of the Rocky movies were good,I feel they devalued the original (a true classic) in someway.
I feel they could have got (or could still get) a great follow up to Who Framed Roger Rabbit.