It was divisive, and that's about it. It still garnered great sales and a decent reception from critics.
It's the Lightning Saga as a whole along with XIV's MASSIVE failure that has really been slowly killing the franchise as of late.
The one most responsible has been XIV, though. Abysmal sales, abysmal reception from both fans and critics, tarnished the brand name, and pushed the series' schedule back a few years, as confirmed by Wada.
Everyone who votes XIII in all honesty has no clue what they're talking about and are just blindly voting because they personally don't like it.
"Dyin' is easy. It's the livin' that's hard..." Grim Reaper, Maximo vs. The Army of Zin
It did kind of destroy it though. Just because you and I like it doesn't mean it's good. Now, you might laugh at that, but Iwhat I got from your post is that the people who didn't like it are wrong. Why can't it be the other way around?
Anyway. Myself, I enjoyed XIII, but I can why its design pissed a lot of people off. It might have sold well, but it did damage the FF brand.