Personally, I can't reconcile it. I simply feel like I can't do it to my Dad. So, I'm doing what I can to hold on until he dies first. Sorry- I know that doesn't help.
I think, like others have said though, it's a sort of balance: My current pain and how much more I can cope with vs. What they will likely go through if I do it.
If the balance tipped for me so I truly thought I couldn't cope anymore, I'd probably do the (possibly foolish) thing of trying to explain to my Dad why he would need to let me go. That way, at least the shock would be reduced and, we could say goodbye. It would really depend on the parent there though. That would likely lead to psyche ward sectioning for some people! I think my Dad would be devastated but, I think he does actually love me enough to not want me to suffer.
It's very hard for you though. A very tough time for all of you. I'm sorry. I really don't know how to get around it. I guess people just get to a point where they can't cope with anymore, no matter what.