This is just speculation on my part, but I think the Prismatic era may be the last era we will see Katy as a "pop-queen or diva". Even on Prism, some of her songs are trending towards mature vocalization, a la Kelly Clarkson and Sara Bareilles. Katy herself has talked about experimenting with different musical styles and genres (e.g.: country), or putting out an all acoustic folk album. She's probably seeing herself more as a musician than a performer.
Also, by the way things are looking, Katy would wrap up the PWT well into the fall of 2015. By that time, Katy would be 30 going on 31. Her audience/fanbase will be a bit more mature, and the younger generation will for the most part be latching onto younger pop stars. It would make more sense for her to attract a more mature audience, in spite of her declaration that she will "never grow up".
Katy may also develop the urge to want to start a family. That will change her perspective on life, as well as the demands on her time.
I would like to see Katy go back to her rocker-chick roots, as I see a natural fit there (I love Fingerprints!). But after the Prismatic era, Katy would have in all likelihood earned the right to do whatever she wants.
And no, I don't think Katy would quit singing. Ever.