They failed to introduce a new main stay in the series that they could use for years. They are always reliant on Chris and Leon. Jill was relagated to a supporting character in RE5, and served as a co-lead in Revelations with Chris. Claire was the given the lead role in Revelations 2 and we know that is not a mainline game. Jill, Claire and Sherry returned in the remakes but those aren't original games. Sheva and Helena haven't been used again. Ada will never be a lead character. They had Ethan for two games as the lead, his face the entire time and killed him for good. While Rose and the ending of the Village (time jump) could screw over future games, as all of the pre-2000 characters would be in their 50s and 60s in the future games (if they appear) and I don't think people would want to see those characters in those ages.
At this point, they need to create characters we could follow for several games or in another 20 years.