Are you comparing UBI recipients to people who get no subsidy/welfare at all? I'm not sure that's a meaningful comparison.

UBI recipients, by the virtue of that "U", are also known as "the entire population". I am a bit confused which "comparing" are you talking about.

one can structure UBI such that utility of income is still steeply increasing at the margin

Can you demonstrate? If you increase the marginal utility of earned income at some level, you will by the same token decrease that marginal utility at some different level. Unless you want UBI to monotonously increase with the amount earned, of course...

people will want to supplement their UBI by doing some work

Humans are satisficers. If UBI is sufficient to pay for a room, an internet connection, and enough pizzas, why should I work? Work takes an awful lot of time, is often unpleasant, the bosses are not the nicest people, etc. Much easier to spend time in front of a screen or hanging out with your friends.

And by the time your low-motivation teenager figures out that money is useful and that advancing in life could be worthwhile, he is in his late 20s and basically unemployable -- not only because of lack of skills, but also because of lack of work ethic.