OP, You are not being unreasonable. It is a big responsibility (as you know) to take on this level of care. I say this as some who did agree to be this type of resource to my NDNs.
I actually don’t regret my decision, and am happy to do it, but wouldn’t necessarily do it for anyone else. My NDN legitimately don’t have any other family, and they have planned carefully for their later years, including a fully accessible house, funds for their care, and making the decision last year to move from the house next door into an independent living community which has step care (skilled nursing, memory care, and end of life hospice) after both had had strokes.
I will also be remotely doing the same for my parents sooner than later and I’ve offered the same to my aunt and uncle who don’t have kids.
I’m up front though, in that if they want my help, they either prepare for services that I’ll arrange and they pay for or they move close to me. My husband and are planning to move in about 7 years and won’t put that on hold. Everyone understands this as well. Our NDNs joke that gives them plenty of time to die. Now what actually happens in 7 years, who knows.
It really does make you think and plan for the future and I strive to as well prepared as my NDN who I truly do consider family. But yeah, it’s not something I would consider lightly.