How does one sub-par sequel destroy the series itself? All the design team did was try something that didn't work.
Conceptually, 1upt tactical combat seemed like an idea worth pursuing (the overwhelming majority here was very excited about it), so I think it's unfair to (with the benefit of hindsight) castigate the team for even attempting it. Now, one might reasonably counter that they're at least guilty of forcing the concept even after it became clear that it was severely compromising core gameplay, but the sheer scope of 1upt's attendant flaws wouldn't have come to light until well along in the development cycle. It's doubtful there would have been time to rebuild from scratch at that point, and so I imagine they had no choice but to simply shore it up to the best of their abilities.
If any parties ought to be criticized, they are management, and to a lesser extent, production. They're the ones who are ultimately choosing to pander to casuals, negotiate unrealistic release dates with their publisher(s), and skimp on beta testing.