I agree that there are exceptions to almost every rule, but the permanence of her condition is unquestionable. As to "functionality" as you say, I think 15 years' clinical experience with Terri, together with the autopsy report and the clinical judgement of most experts,... overwhelming for those who can see it. I can't say that I'd do everything that MS did, but personally I'd feel much worse keeping her "alive" artificially in her condition than allowing her to pass. I'm sorry to say again that the dehydration argument doesn't wash, nor does it change meaningfully anything I have said.
Other people have recovered from conditions that were, by every appearance, just as bad.
Also, I would posit that if you treated a newborn the way Michael treated Terri, after 15 years that child's apparent mental state wouldn't be much better than Terri's.