I only watched the series when it originally aired and have never rewatched since it went off the air. So I can't speak to when it got worse. However, over the years of watching it, I remember being less and less inclined to watch, especially the last couple seasons. It's just one of those things where every show has an expiration date, but networks don't know when to end a show. It's not about quality to them. It's about milking it for everything they can. Fox sucked every dime out of that show until it was no longer profitable for them.

I don't believe any show can benefit from being on the air for 10+ seasons. It's just too much. The characters get stale and the storylines start to run out. You end up repeating plot lines and going around and around in circles. People think that procedurals have the opportunity to go on for longer than the average show because it's "case of the week" and there are endless possibilities, but truthfully, there are only so many cases you can do that are truly that interesting. And there are only so many obstacles you can put a romantic relationship through before it just starts becoming unbelievable and you start questioning why those people are still together if the only thing they ever do is fight, bicker, and never see eye to eye on any situation.

The show ran its course, but Fox was still profiting from it. So they kept it going. I think 6 seasons is a good ending point for any TV show.