Leaving aside that older people aren't as enthused about FT work they've known for 30 years, many of them keep jobs or get bored and lonely without them. Not everyone wants to sit around crafting useless projects, or consuming. Creating value for others and connecting (and status signaling) can provide meaning, and granted in retirement there are other vectors for that made available such as volunteering.
It's not a good argument against UBI, but there are better arguments against UBI.
It's not a good argument against UBI, but there are better arguments against UBI.