I care a little. I'm more a Ravenloft fan so it was always tangental to me. (However, the old cosmology was better for Ravenloft).
However, I know that for many people, Planescape was their favourite. It was their favourite setting. And there are those who love the 1e-3e lore of the worlds and planes.
So why take away someone's favourite world? Why take away someone's favourite bit of the game? Even if they're the vast minority, even if their views are opposite mine, it doesn't affect me and brings them joy. Let them keep it.