I strongly disagree that getting diagnosed causes worse outcomes in such cases.

I'm not sure what the link is between your brother getting diagnosed, and him having the option to skip out on socializing. Undiagnosed people frequently make the same choice under worse circumstances, since it's hard to name why they want to choose this escape. I wouldn't be surprised if the undiagnosed made this choice more often, as they struggle in ways most people around them don't seem to, which naturally leads to the struggling seeking community in other places (= digital ones).

It may well be that a life journey like your grandfather's was more likely for mildly autistic people 50 years ago (though I'd urge you not to forget that many in that situation just break and die early), but the world has fundamentally shifted since then. IMO not getting diagnosed wouldn't improve things, people would largely live the same, but with much greater self-doubt, anger and broken relationships.

it depends what you do with the diagnosis. Once you have that label, does it make it an excuse or an explanation for the behavior. Do you go "I'm autistic so I get a pass" or is it "I know I'm autistic so I have to work harder just to keep up with everyone."