Lynn Goldsmith sold the photograph, with full rights, to Vanity Fair, which then commissioned Warhol to make the Prince artwork out of it.
Then, when Vanity Fair started doing stuff with it again in 2016, Goldsmith saw a chance to cash in, and sued...not Vanity Fair, no; the Warhol foundation, which guarantees a much higher-profile case, and presumably has more money.
This should, by all sanity, be, a simple, cut and dried contract case: Did the sale of the photograph to Vanity Fair include the (transferable) right to make derivative works, or did it not?
If the Supreme Court makes a ruling that affects Fair Use on this in any way, it's 100% because they wanted an excuse to do so. They have a very easy route to ruling on much narrower grounds, or kicking the case back to the lower courts.
If it didn't, then the fair use argument and decision is important.
Unless there's an ulterior motive like trying to set a precedent to solidify the position of other works on shakier legal foundations, I don't know why the Warhol Foundation wouldn't take such a clear-cut defense if they actually owned those rights.
Warhol deserves to lose, I put the odds at 85% Warhol will lose, and I'm doing my best not to lose my sanity with so many misinformed or willfully ignorant (Mike Masnick) to argue feelings versus facts. There are many more good Fair Use cases than this one, and that should start out every discussion or essay. It gets clicks, I understand, but it's also a game of whack-a-mole for me, because it's so pervasive.
I should've re-taken the LSAT and gotten that full ride to Pepperdine to pursue IP law...oh well...passion never dies. It's still relevant to my interests as a musician and creator. There are rules, Smokey...