>> You are free to support those who can't meaningfully contribute, but where do you get the moral authority to demand that I do so as well, with my own money?

We're not quite there yet but what happens when all the food and goods a society need can be made by automation, run by a skeleton crew of (say) 5% of the population in facilities owned by 1% of the population? Nobody else can compete on price or availability, the makers don't need staff, and only so many can fit into the service industries.

Do the 1% continue to take the majority of wealth as rent, and the 5% live reasonably well, and f*ck everyone else? Or do we recognise that human progress has been a group effort and start to share the abundance with everyone?

Besides which, if you're asking where the moral authority to demand you help house feed and clothe people comes from... LOL. It's already here. What's not moral is the immature libertarian "I've got mine" attitude we see so much of these days. Tax to help the less fortunate and societal development is theft of my stuff by force and inherently immoral? That's a pretty stunted and selfish thought pattern right there.

>> isn't there a good chance that a permanent underclass with a guaranteed welfare income will overpopulate in relation to those who are "meaningfully contributing", to the point where the ratio is so bad that the system breaks down?

Oh no, the poor people will breed!