The thing that fucked up horror films is the same thing that fucked up Hollywood in general. An unwillingness to take risks.
The two classic eras of horror were the 1920's - prior to the dominance of the studio system - and the 1970's - after its demise and before it reestablished itself.
The basic problem is one of economics v aesthetics.
A film that has a happy ending will make shitloads of money at the box-office but will be seen as lightweight and will be largely ignored by future generations.
A film that has a bleak, ambiguous, or daring ending will garner critical acclaim but less bums on seats in its opening month.
This is independent of how good the script / performances / effects are. And guess which the studios go for. Meh.
No studio would make Rosemary's baby or The Exorcist today - well they would, but they would bolt on an upbeat ending (or at best, an upbeat ending with a tiny coda hinting at a sequel.) Formula is king - cos formula is safe. McG rules Hollywood.
Most of the best films, of any genre, these days don't come out of Hollywood. They are produced independently or in other countries. There are decent films out there. Pan's Labyrinth is comparable for shocks with anything I have ever seen - mostly the non-fantasy sections. The asian market has produced some great horror - Ring, Eye, Grudge - which have been lamely remade in tinseltown.