Heard that one before.
I disagree however in cases of severe disability it is something that should be thought over very carefully. It is very difficult being the daughter or son of an adult (or two) who are severely disabled or even moderately (as I would say my mum is).. I have done things at 22 (and have done some level of caring since age 3) for my mum that most of my peers hopefully won't have to for another fifty years or so - including monitoring medication, changing clothing, bathing, putting to bed and managing continence needs, attending medical appointments and advocacy, managing finance, signing care plans etc..
You have the emotional impact as well - the not wanting to go out, not being able to relax at school, not being able to have sleepovers as often, lower disposable income or no reliable income (e.g. benefit reliance), etc.
However - there are huge upsides. I'm good with first aid, very used to hospitals and ambulances, can to some extent keep a level head when things are shit, used to working under pressure etc..
It's a decision that would need to be weighed up very carefully - just as it ought to be all the time. You'd need to have a clear idea of whether there would be support for your children through family, friends, young carers networks, etc. In the most severe cases you'd possibly have to be prepared to accept a lot of intervention at home - I have been cared for as a child by many people, which I know my mum found hard.
I don't think it's selfish and it's not wrong, but it does need to be thought out.
However, it is absolutely disgusting to say to a parent with a disability at they are selfish. Some people are just ignorant twunts.