I hadn't picked up on the suggestion she is immortal until shivver13 mentioned it above and I agree it is odd. I still think we are supposed to consider this a fate that is better than death - so maybe the Davies thought immortality would be a plus (even though WTF?). In principle I don't think Ursula would have been better off dead than as (essentially) a paraplegic, but I agree that some of the way it is framed is distinctly peculiar and a little distasteful.