Well, slaes only tell part of the story. I mean look at how the platform genre has evolved since the release of J&D. aweosome platformers havbe been released, some of which are near perfection both technically and in gameplay, still, sales are not as high as one would expect.
In the same fashion, should the survival-horror genre get a much deserved "evolution", there is the risk that whatever developers do, sales won't reflect the effort.
If the public has had enough of something, it won't buy it. Of course there are exceptions though.
Think about it, which one of the 2 (SH2 and SH3) should have sold more? My bet would have been SH3, much more in-your-face and commerciable than the highly psychologic SH2. However, SH2 sold more. The market is already showing that we are buying less and less survival horror titles, as good as they are.
Personally i will always buy SH games cause i just love everything about them, but thinking a 4th sequel (as good as it might be) will suddenly boost sales of an already milked franchise is a bit optimistic.