I totally get it.

We had our health too, about 15 years ago. Things change as we age, often through no fault of our own. Don't count on living forever.

I understand the difficulty naming a guardian, too. Nobody will ever be as good as a parent, ya know? When we were going through this process someone said to me, "Who would you want called if you suddenly died and the choice was your GUARDIAN or FOSTER CARE?" Holy cow! That put in in a whole new light. The guardian we ultimately chose is not the warm, fuzzy person that I am but she will be diligent about his care and I have no doubt that Christian will always receive the attention he deserves.
This is great advice. We made our wills about 7 years ago, about the time that DH first got real sick. My father died intestate(without a valid will) and it was a mess. It really opened my eyes to the REAL MESS we'd have if both of us died without establishing guardians, and such.
For our situation, we decided to have separate guardians for our chidren. Christian is such a lot of work that I really felt it would be unfair to expect someone to take on our other kids plus him. Now that DD17 is almost an adult and DS23 has his own place, it's not as big an issue. However, we still have a guardian for Christian. We do not expect this guardian to be able to physically take care of him and we have talked with her about placement(*assuming he's not already placed) She will have the power to manage his finances too.
The elder law attorney that we used helped us set up a Special Needs Trust for him and it will be fully funded at our death. We have changed our beneficiaries on our life insurance to "Special Needs Trust Fund for Christian___". The other kids will get smaller amounts, plus the house. We've tried to explain our decisions to the kids, but I'm not sure they really understand it.
I don't want them to think we loved them less. In fact, because we love them so much we don't want to leave them financially or physically responsible for Christian. Since they wll have he ability to make their way in the world and he will not, we want to make sure that it doesn't fall to his sister and brother to pay his way.