With McCartney, The Rolling Stones, Stevie and maybe not so much with Bowie, people perceive those artists as old so their new stuff is really isn't thought of as serious art.
With Prince, since Prince comes off kind of youthful, people are like: "Why isn't Prince's new stuff as impressive as his 80's stuff?". So Prince seeming youthful makes people who buy and listen to his music more demanding that it be up to par.