Honestly you are underesteameting both of them. Prince also never tried to be as big as MJ. but in a world with no MJ. Prince who was at the start of his climb in 84 would end of being marketed even more in this time line. Only other thought is someone like L L Cool J becomes a bigger cross over Rap and Pop star or Bobby Brown for R&B and Pop
As you say, Prince never tried to be MJ -- he also never wanted to be. A world where Warner tries to make him the new MJ is a world where Prince and the label have a major falling out many years earlier. The breakdown might begin very quickly -- 1985's "Around the World in a Day" was pretty out-there and very much
not the sort of thing that Warner wanted to follow-up "Purple Rain."
This would probably be a huge boost to Madonna, though. She was already going platinum and scoring top 10s with her first album, on the verge of releasing her massive second album. In OTL, she was the only artist in the world who could compete with MJ in sales and celebrity, and now basically all of MTV has been ceded to her.
I fear Janet's career may, counterintuitively, be much weaker in ATL. She's yet to have her breakout hit with "Control" and I imagine A&M may want to throw money at Quincy Jones to try and make her the next Michael. This may get her a hit album or two in 1985-7, but I doubt it has much appeal in the long run. Without Jimmy Jam and Terry Lewis in 1986, her whole career is turned upside down.