I know Resident Evil has changed drastically since it began, but that's the nature of story telling over a long period of time. You can't still be doing the same "surviving against the unknown" thing for too long because at some point your characters are going to learn and adapt and the story has to progress.
The is a very indepth story going on with Resident Evil by the time RE4 came out, hell even before that, but the first three main RE games were set in a time that were so close together and RE2 and 3 overlapped each other. In 4 and 6 the characters have grown and the story has progressed. Leon is working with the government to deal with the bio hazard threats, Jill is working for non profit organisations to save peoples lives who are effected while Chris is working for an official group that deals with it. Then in the shadow's you have Wesker who has been plotting something since before the events of Resident Evil and Resident Evil Zero and Ada who is another complete mystery in the series.
I mean, it's no wonder that there is no horror anymore because part of being scared is not knowing what you're up against... but now that things have progressed this far, the characters and the player already know pretty much all there is to know about zombies and ganado's and the different virus typs T, Veronica T, G, etc... and the parasites and all that...
Also, they can't just end Resident Evil at this point either... there are way too many open ended things going on which they haven't tied up at all... the show must go on, we have to find out how this truely ends, and I do think at some point in the future the series will come to a definitive end.