Cities are economically efficient. If your praise for UBI is that it reduces economic efficiency (reducing your ability to pay for UBI) then I think you're going off course.
No, paying poor people to compete with each other for city space is not efficient. In terms of efficiency, it's about on par with stacking cash in a pile and burning it.
City space is finite. We ration it for a reason. We could ration it differently, but we'd still have to ration it. Increasing density doesn't make it affordable to pack everyone in a single place (see: NY). Pretending the problem doesn't exist and throwing money at housing projects doesn't make it affordable to pack everyone in a single place (see: SF). The only way to make progress on the actual problem is to figure out how to deal better with being spread out.
Increasing density helps a lot. The US mostly has lost the skills and will to do density. New York mostly stopped building enough to keep up with demand.
Look at Asia to see how to tackle these problems better. Eg Singapore.
Yes, it's not completely clear whether they'll have the political stomach to actually go through with letting the leases lapse at the end of their life.
It would probably have been better to use Land Value Taxes instead of relying on limited leases.
City space is finite. We ration it for a reason. We could ration it differently, but we'd still have to ration it. Increasing density doesn't make it affordable to pack everyone in a single place (see: NY). Pretending the problem doesn't exist and throwing money at housing projects doesn't make it affordable to pack everyone in a single place (see: SF). The only way to make progress on the actual problem is to figure out how to deal better with being spread out.