I have mentally prepared to see my dad grow old and die as a lonely old man and I can live with it.

I am living this dream -- similar situation, I'll spare you the long story. Dad just died a few months ago, after a LONG lifetime of bankrupting himself, mom, and several other family members, and he never changed. He never even regarded himself as having a problem, just dismissed his massive problems as bad luck and criticized family as unhelpful when we imposed any boundaries at all. In the end, the universal feeling upon his death was relief, and there was just a lot of uncomfortable silence and not too many people at the sad little zoom memorial. Some people are just like that, and absolutely will not change. You can feel bad for saving yourself, but don't not save yourself.