Actually, I don't have a problem with its costuming/special effects. I'm a classic fan, too. I didn't like the mechanics of the monster, and the faces of its consumed victims appearing on its body, or the fact that they still had consciousness and could influence what it did.

I look at it very differently. To me Ursula's statement that she doesn't age implies also that she'll never die. She'll be locked in that stone slab after Elton has gone. In a way, it's similar to Borusa's fate, and certainly implied to not be a happy one. Part of that, of course, is because Borusa asked for immortality without knowing what he was really getting, but then the Doctors, given the same offer but knowing what it meant, unequivocally refused it.

Maybe it's a question of choice - the Doctor didn't give her a choice. Perhaps he did afterwards, telling her he could let her go or she could stay in that slab forever, and she chose to stay. But to me, it's a horrible fate, and it surprises me that the Doctor, who understands that fate all too well, would even offer it.