Femme Fatale was a successful era, I don't really see the point of discussing this. The album underperformed, sure, but not like it wasn't selling at all, she had a few hits in certain markets, the GP was aware of her being alive, she managed to go on a WW tour, like...
I know y'all expect everyone now to do Tayloose/Adull numbers and if not then the era is a BOMB, but FF did just fine for Britney. At least for anyone that wasn't expecting too much, especially that she looked and acted like a lazy pig most of the time. She's already passed the point of being on top of the game, she has her legacy to fall back on and now it's cool if she can have an era which would be considered an "underperformance" for someone like idk Katy Perry.
I mean I hope she can have a few more moments at the top but it's not like she's gonna come slaying left and right. Of course she has a lot of fans that still count on it and a lot of haters that are afraid of it. After all, she's a legend
She had a lot of the comeback hype (see HIAM's first week sales) and much of her fan base who were going through Britlexia, but she disappointed people. The music was there, and the album sold solely based off of its music, but that's not enough since Britney is a superficial pop star. People wanted to see good performances, her looking good, etc. She was still a big celebrity then, so for her to sell less than 2m worldwide was pretty bad. And the tour did okay...same with the singles. 
