Here's what went wrong with the later seasons. During season 5, we had writers like M.A. Larson, Amy Keating Rogers, Megan McCarthy, etc. These writers wrote more good episodes than bad. In season 6, they were all replaced by writers who had little experience and passion with the lore. It's no coincidence that what was objectively the best season, was followed by the objectively worst season. Another factor that two of the later seasons had Josh Haber as the story editor, and it's also no coincidence that those seasons were the most hated. Plus the fact that the Lady Writers were missing in season 8, yet another factor of what went wrong with season 8.
Had all the good writers from season 5 stayed longer, the later seasons would've been way better. But that's what happens when you don't have commitment in the writing department.