They're removing black to force people to either upgrade, or make it painfully obvious to their peers that they're using an old phone. That's what you do when there is nothing new in your phone this year. Status signaling and fashion.

Or it could be like the white iPhone 4... delayed because of problems with the finish.

It's obvious to anyone given the radical new design that you have an iPhone 16 Pro or not. That's not the reason for removing black. They'd have removed silver for the same reason if it was.