I'd agree w/you that it seems like a waste for someone to drink away their life, their opportunities, their strength, their health, their smarts, their love. It seems like a terrible waste. I felt (and still feel) that way about XAH.
But, to quote Gandalf, "even the very wise cannot see all ends." You don't know what his future holds or what path he will travel to arrive there. I think all you can be sure of at this point is that staying with him would have caused you to waste your life, your opportunities, your strength, your health, your smarts, and your love.
People here advised me to imagine wrapping XAH in a blanket and turning him over to his Higher Power. That image felt too much like death to me and didn't work well, but I've since found other ways to imagine letting him go into the care of something/someone much greater than I, a power that will take care of him in ways I never could. That does work for me. Maybe it will for you too.