There was a time when Ronda Rousey was the best fighter in Women's MMA. In the context of her entire career she was clearly not the best ever. She overestimated her striking and got KO'd by Holly Holmes where she lost her confidence / mojo. Then she literally got fed to the lions when she came back to fight Nunes. She was clearly not confident and wilted as soon as she started getting touched up. I'm sure she took some damage during her career, but I think her fall was more mental than the neurological excuses she kept trotting out.
Getting blasted has caused many fighters to lose their edge. Weidman, Rockhold, Barao, Till, Hendricks, Reyes, Walker, Moraes, Garbrandt, Ferguson and many others. Some fighters have come back from adversity with later career resurgences like Oliveira, but most don't. The stress of being a long running champion and fighting the top contenders on a regular basis can wear you down. GSP was smart to take time off and eventually had a successful comeback fight for a second weight class belt. There are many reasons why there are so few champions who put together long championship winning streaks. For every Jones, Silva, GSP, DJ, Aldo, etc. there are Rockhold's and Garbrandt's.