Her popularity is at a unreachable point. She has long since met her level of potential. To expect a musician to increase the quality of their music every single album is pure idiocy. You cant constantly just get better and better.
You can also only be at your peak so long before you stop being the darling. When people love an entertainer at such a high degree eventually they get bored and will turn on them.
At this point Taylor swift isn't popular because of her musical talents, she is only popular for being Taylor swift. She isn't even a person anymore to her fans.
I'm no fan though. I couldn't pick her out of a crowd or name one song to save my life. I never liked her music because she is just some product to be sold to me. She is a good singer but has nothing I find value in.
She isn't a musician, or a writer to me. She is just a performer performing stuff deemed marketable to as many people as possible. She has nothing unique or special from what I can tell. She is to music what marvel/Disney is to movies.