The comparison to boy bands/hair bands is silly, you're WAY into "get off my dad blasted lawn" territory. A couple things make TS defining for so many girls and young women. It's clear she's an enormously talented lyricist, and her diaristic, autobiographical style really speaks to the situation of young women, with depth and not just "I like this boy and he doesn't like me". And she's been successful for so long that her music tracks (so far) from her teenage years into her 30s, so her music resonates with the lives of girls as they transition to young adulthood and maturity. For more than one generation now, she's been the main musical influence as girls grow up and move between life stages (she's had 10 studio albums over 17 years and all but the first reached #1 on the Billboard charts, and her most recent was one of the most popular albums of all times, as she gets older, she gets MORE popular). You might not feel her music has substance, but that's because it doesn't speak to your situation as an older man, she's speaking a language you don't know and talking about problems that you don't understand.
If you think her music has "no grit", then you've clearly never listened to it. I don't personally like Anti-Hero (don't tell my daughter) but you don't get a lot more grit than that. The 10 minute version of All Too Well is full on grit. I don't know most of her music but the "no grit" claim is a dead giveaway of someone who has no idea what he's talking about.
BTW, have you seen how many colleges are now offering classes on her music? How many colleges had classes on the music of Boys to Men? I'm guessing not many.