Oh boy, I think I can help you here. My wife became a huge swiftie part way through my being with her, and I'm a musician myself, so I've ended up accidentally digesting a whole lot of her music extremely thoroughly when I normally would not have.

I already don't listen to the radio, but when I'd heard her songs before I'd casually enjoyed them as catchy hits, but I have come to respect her quite a bit as an artist and genuinely enjoy a good few of her songs. There could be a whole documentary on her success and why, but I'll try to sum it up as best as I understand through accidentally learning the ways of her fans and analyzing her music, even trying to be succint there are a few big points to hit on here, and this will be stupid long, probably two comments.

1. Her music is very well written and mixed/produced/performed. On its own that doesn't really matter much in the sea of pop artists, but her secret weapon in terms of retaining fans is her variety. Basically, this is what allows the "eras tour" to exist. She started country rock, became synthy pop, rebooted into indie rock, and has now transitioned back into electronic pop, but with a more laid-back vibe than her excitable pop anthem days.

Her songs are mostly very smartly written pop, but her band focused music also has some smart instrumental writing and performances for people who enjoy music more than just for pop catchiness. For instance, the song I most enjoy by her is Tolerate It, which is written in a 5/4 time signature, which is very unusual and interesting for pop music, but the song is so smartly written and the vocals are delivered in a catchy enough way that the listener doesn't even really notice the strange time signature.

To top this point off, her lyrics can be quite good. Mostly, it's just well-written pop again, but on some of her songs, and particularly across the indie rock styled albums Folklore and Evermore the lyrics can be really excellently written and poetic. The subjects of love and loss she predominantly writes about are generally relatable by everyone, and probably very relatable to some, but she also writes about other things on and off, like gender inequality or the loss of family, other very relatable topics for people in general.

2. Her start in country rock was very down to earth and her immediate talent in songwriting out the gate established a strong cult following early on. Again, this is not a very unique thing for pop artists, but her secret weapon here is that she has retained a very down to earth persona and a feeling of honesty with her fans, which is bolstered by the fact that she writes her own music.

She collaborates with artists, too, but she definitely also has a very strong "author's voice", and if you listen to and analyze enough of her music you can come to understand some of her unique melodic touches and bits of her songwriting that are throughout her whole catalog. This is why I, as a musician, respect her the most. If she just took songs from others it wouldn't be the same thing. It's a bit like Michael Jackson and how he would really give it his all for songs and his personal touch vocally always came through, no matter the genre style he was performing.

3. She's extremely smart and doesn't breed controversy. If you listen to interviews with her or really hear her talk about anything in depth at all you learn that she knows a shitload of stuff about a lot of things and is very well spoken. She's whip smart and that no doubt has significantly contributed to her success on a business side. She also doesn't do anything evil or controversial aside from the regular mass production of merch and usage of private jets, etc.. her biggest controversies were writing too many breakup songs that got popular, which, compared to many other artists is not even ranking as controversy. For someone who's been in the game for this long that's a big deal for staying power and loyalty. (Continued in second comment)