I think I get both sides of the debate. Sort of.
I've been a carer to close family several times. I had, and have, zero resentment for them because it was 100% not what they would've chosen for themselves or me had they ever had a choice at all. I don't regret what I did for them in the slightest, and in fact only wish I could've done more, and better.
That said, man. I'm still not entirely back to being a whole person since the last time. To say that it was brutal and detrimental to me is a severe understatement. I did it for family, if not for them, for whom? If not me, who? There was a time when I wouldn't have written someone off as a partner due to them requiring extra assistance, provided it was a legitimate need rather than a vampiric personality (perhaps the MIL Cheryl describes has that personality type on top of her objectively unfavorable condition, and there is behavior that family resent, rather than resenting her actual disability). However, some time ago, I still had some capacity to assist people to a considerate degree. That's gone now, for real. One, I've been a carer to people with extreme need for assistance, in some other past personal relationships I've been a codependent caretaker, so I'm, uhh, done, and two, I occupationally assist people in multiple ways. There's no way I could assign myself as caretaker or caregiver to a partner these days. Not so much an emotional reservation I think, at least insofar as we're talking about disability without a problematic character type, just an honest statement of how much I can extend myself these days without ending up hospitalized myself, or how psychologically drained I'd get in no time. I think many people would be equally likely to exhaust themselves to extremes this way, even more so if marked codependency tendencies are present, as they often are.
While one can partner up with a healthy person who ends up needing care later on, that's certainly not the same to me as choosing someone who needs the care now, and is only going to need more as time goes on. The only way I could personality pull it off is if them or I could afford long term outsourcing.
In OP's case though, I'd urge him to leave now. There was a thread on here last time I was on that pissed me off to no end. Guy dated, married, and twice impregnated a woman whom he knew from the start had several serious conditions that rendered her a suffering ruin (one of those would've been enough to render most people the same), and here he was, twelve years later, complaining of her STILL not being a perky, energetic, functional joy. For twelve frikkin years he acted like a disabled person should've acted and functioned as if not one thing was objectively crushing her to rock bottom. Why not just date, marry, and impregnate an amputee, then complain she never goes running with you! If this is what Cheryl suggests by warning OP to realistically consider the extent to which an ill person's needs tend to grow, I totally get that. Don't be the guy who later complains she's not some other, healthy woman. In no way do I get people who chose an ill person, and then complain of not getting a healthy one. And yes, as Sherry points out, those are terribly detrimental to the disabled.
OP's concern of hurting her by leaving is a true portrayal of how clueless people can be of a disabled person's experience (at his age, I can understand). It is cruel to stay with her while resenting her condition. I guarantee she feels every bit of his disappointment (he's already resenting her for not being "exciting" - he is disappointed in how boring a disabled person's life is. Until he accepts what they've had to accept, that they won't hike, party hard, work a challenging job, energetically entertain their children or even change their clothes, no, he shouldn't be staying with her)--
and she's young enough for this to leave a lasting influence on how she feels about and deals with her condition and relates to others, and she already has previous experience that requires a change in who and how she chooses to get involved with in order to not cause herself permanent discouragement in living and relating to self or others with her condition.
If he's feeling big, perhaps volunteering to help the disabled would be a better introduction into the lives of disabled, given the number of other people assisting, gradual and moderate taking on of chores and exposure, and the chance to get an understanding of their experience without his personal needs and expectations obscuring it.
This girl will likely have a painful personal journey learning to grow despite and through her limitations. I wouldn't personally want to be the person behind her lone eventual realization the tremendous task of nourishment, encouragement, and guiltless acceptance is most likely to be best undertaken by herself and a competent, compassionate professional or two. Life has a way of sending the right partner along when one learns to do without, and certainly a way of teaching one to stand alone when they would most desire to lean on.