I'll argue with the idea that this marks the beginning of the series going downhill again. Looking at the original series, a lot of people consider 2 to be the best. This one is being directed by the same guy who worked on 2, and I think the new direction Resident Evil 4 took the series in is still fresh enough to make this game fun. So I'd say this could be the best game in the series, or at least an equal for Resident Evil 4.
What it probably does mark is the beginning of the series being over-sequelled again. This one could turn out great, but when 6, 7 and 8 follow on an annual basis, it will get boring again.