While there is always the potential and sometimes realization that the quality of any series can diminish over time, this topic is about Resident Evil and what changes could be made to a future game in the series such that it would be good. And really I can name at least one series that has done worse than Resident Evil in terms of later games in the series - Sonic, but that is not what the topic is about.
And really though, as long as people are paying for the games, then there is no real need to stop making them; it might even bring in some revenue to attempt a new game; budgets tend to be bigger now so money helps. Capcom might make some changes to the newer game's but yeah, if they didn't attempt anything new, for better or worse, it'd just be the same game over and over. This actually works for a while because graphics can improve older games and whatnot.