I think Taylor Swift is a unique case. Not unrepeatable, just currently unique.
For one she is genuinely talented. Many pretty singers are in fact terrible song writers and mediocre musicians. Their hits are often written by middle aged men, occasionally middle aged women, and rarely themselves and pulled from fake books.
Taylor Swift can write music, sing, dance, and even play her music so she is in rare company with those talents. It helps her a great deal she is attractive. There's plenty of talented women in music that aren't/haven't been as successful because they aren't "pretty".
The second part of her uniqueness is she came up in an age of peak media saturation and peak ease of distribution. When Taylor Swift started the Internet was already big and she had a significant online following. iTunes ascension was also happening so she could easily sell to people only wanting singles rather than whole CDs which maximized her sales under the price/demand curve. P2P file trading was also prominent so she got even more space under the price/demand curve.
The third part of her uniqueness is her age. She's positioned right between Millennials and Gen Z which are both pretty large demographic cohorts. Even her early work was fairly mature (considering her biological age) so it didn't just appeal to people her age. If she was ten years older I don't think she'd have her current level of impact, it would be significant but might not straddle generations so she'd have far fewer younger fans.
The situation in the 50s and 60s was very different. Distribution was harder and more expensive, there were fewer radio stations, and music on TV was very limited. Not to mention just fewer radios and TVs. A household might have one of each but it wouldn't be until the later 60s you might expect a teenager to have their own record player or radio in their bedroom.
By the time of Taylor Swift it would be uncommon for a middle class teenager to not have at least their own radio. A non-trivial segment had their own computers by then so could easily access Taylor Swift's online presence, fan-based or direct.
So I'm not sure her appeal to multiple age brackets being the overall industry coming full circle but instead some unique factors of Taylor Swift individually.