1 is of course the only actually sustainable solution, but I'm trying to say that even 1 isn't completely smooth sailing in an aging society even when you get rid of capitalism. Or, rather, people are going to have to accept a lower standard of life.

If you get rid of the capitalist leeches, yes, you have more workers left over because there's no more demand for yachts and other shit. But really, it's only the luxury goods that demand will go away for. The rich hold nearly all of the purchasing power in the world, and they own a bunch of land and other assets, but for the most part their wealth is still on paper, not in tangible usable goods they've bought. Elon Musk COULD liquidate all his stock, but firstly he'd lose at least half of it due to the massive value drop when he sells so much Tesla stock at once, and then if he tries to spend it all on, say, rice or something, he'll find that there's a limit to how much rice is actually produced, and he literally couldn't spend all of it at the current market price of rice, without a bunch of new rice production happening first.

There's still so much in the world that gets made or maintained by human labour, that we take for granted. From food, to working plumbing, to medical supplies. Unless we can ALSO automate production of most things we consume, we still need to have a bunch of young people working.

This is not to say that I support capitalism as the best economic system. It's far from it, and billionaires shouldn't exist. But at the end of the day, we still need people to do jobs. More equality in the distribution of resources doesn't mean we suddenly get said resources without any work. It just means we have less bullshit work (building yachts and skyscrapers, anything to do with stock trading, etc), but I think most people overestimate the share of bullshit work in a healthy modern society (the US does NOT count as a healthy one).

Of course the irony is that if we manage to automate the production of (nearly) everything and there's truly no more need for anyone to work, young people might start having more children again and there'll be more people who could work.

I also don't think there's a need for UBP if there's already UBI. The U implies nobody is left behind. If you work, you get UBI, if you don't work, you still get UBI. If you're 120 years old, you still get UBI.

As for your idea #3, that's just unfair towards the people who have to work. The idea of UBI is that everyone's taken care of, but those who work can afford more nice things. If you don't do UBI, but instead do "communism for old people", that means that young people have to work to even have food, whereas the old people just get to enjoy the spoils of young people's labour. UBI is more fair, in that those who put more effort into society still get more. If society is productive enough, UBI could be big enough that those who don't work can also have nice things (like travelling). I'd say that for sustainability and fairness, it has to be either UBI, or communism for all, but not "communism for one part of society".