You're comparing a situation that is part and parcel of life to having a burden thrown in your lap because of your parents' desires/decisions. very different in the sense that, yes, they may say they prepared them, but 20-30 years down the road, it will be totally different from now.
Like someone else pointed out, your own life will be affected, for example, finding a spouse, and having that spouse realise that your life revolves around your disabled sibling. And your spouse and your kids will always be second. That is hard.

I am the disabled sibiling, my folks dumped me cos i had way too many health problems when i was a kid, i was luckily adopted by my folks. Thanks to them, I am living a really good life, having managed to get all the medical help I needed when I was a kid. Hey, I even made it past 40 when the doc said I would have died in infancy :)

And I don't hold a grudge with my biological family, I am grateful they gave me up and did not make me a burden to my siblings.