If we implement BI today, most people will keep their jobs out of fear it'll be repealed when it's unsustainable.

I disagree. I think many people would quit their jobs immediately, knowing that they'd receive a comparable amount of money without having to go to work at all. Nevermind the fact that they could keep their jobs and make twice as much money. Enough people would throw their hands up and say "fuck it!" that we'd have a serious disruption of business, were this implemented now. No, I don't subscribe to the belief that all or even most members of lower socioeconomic status don't work hard or don't want to work hard. But I think there are more of those folks than Hubski is willing to acknowledge.

Twenty years from now, the next generation grows up expecting BI, and we have say 20% less workforce, which is counterbalanced by increased automation.

That 20% estimate seems high to me. Were we to immediately replace all truckers, cab drivers, food delivery service workers, etc. with self-driving automobiles, that's only ~5% of the national workforce. And the transition won't be immediate. There will be other automations taking place concurrently, sure, but I'd wager that we'll only see a 10% workforce reduction for the next twenty years. Then maybe 15% for the twenty years after that, and then maybe 20%, but who knows what things will look like 40 years from now?

Fast forward 100 years, and we see a logarithmic decrease of involuntary work and increase of automation, until we achieve Star Trek utopia. That's the theory anyway. If it doesn't work, we can always repeal it, right?

Pshh, I hope that's how it'll go. And I don't expect for basic income to work at first. It will be a rough process that could send the global economy into free fall. I'm interested in making the transition as smooth as possible, because it does seem increasingly necessary.