But she hasn't "declined" though.
In fact she has been one of, if not the most, consistent artists in the industry for almost 2 DECADES. That is a HUGE feat in itself.
Also not to mention, she has actually grown BIGGER with each era, which is why her so called "decline" in sales, doesn't really matter that much.
No matter if you like her or hate her, NO ONE can deny, that today in Pop Culture, there is not a bigger name than Beyonce. This is especially remarkable given the amount of time she has been in the industry. Many artists cannot keep this kind of momentum for even over a decade, let alone almost 2 decades.
There's something to be said about that. Which is why her fans "act like she doesn't decline", because well, she really doesn't.