Katy is huge still because the hits were humungous and constant for a key time. Kids will likely know her songs as classics growing up and her videography is one of the strongest of her class.
Like she's a name. Once you reach a certain level of name recognition you will always kind of sit at that level of famous. It will get bigger but not likely smaller. And PLENTY of other acts have managed to carve long careers well past their biggest hits because back catalogue/name go a long way vs a rogue TikTok number 1.
She really did her thing on some of those Witness album tracks: the title track (especially), "Déjà Vu", "Power" and "Tsunami." Amazing tour, too, even if it was clear her former dominance was running on fumes. I kept my Karma Coffee & Tea receipt!