Yes I am still a proponent. Though I don't think it's as simple as the economics simply adjusting to negate the benefits. I don't think that's true at all.
When people have a different quality of life and certainty about basics, that doesn't just affect those people. It affects other people throughout the rest of the economy too.
People who are poor or very poor often contribute a lot to the economy just like many other people. It just doesn't get the same recognition. Much of the work is invisible to people not actively looking for it. Probably the most clear examples are carers: Caring for their parents, their children, their ill, disabled and elderly friends and relatives. An astonishing amount of labour is performed by people who aren't paid to do it, sacrifice a lot to do it (e.g. many have little time for socialising and friends), and go largely unrecognised (after all they don't get out much, its unpaid, and its anything up to 24x7). A fair amount of cash or cash-equivalent is provided by the poor to the even poorer too, e.g. bringing over food, providing accomodation in a spare room.
I think the quality and level of the overall contribution to the general economy would improve with UBI ensuring some measure of stability and peace to people at the low end, or who are facing being at the low end (e.g. redundancy during COVID-19...). I also think there is a moral imperative to provide the basics for people if it's possible; a "dignity of every person" sort of thing.
I've seen what it's like for people who also have "welfare poverty", that is they can't access welfare reliably either. And I have seen people live on less than one meal a day for months as a result, saving their limited food stocks for good reason, and with no idea when their precarious housing will come to an abrupt end, and no bed to sleep on where they live, in a rich Western country.
I don't think it's welfare per se that causes the stressors mentioned, but rather the complex fear and uncertainties that are rightly associated with the common versions of it.
With regards to whether poverty should be seen as the cause, I think it is, but the degree of stress depends what kind of poverty you mean.
Having a low level of income that is absolutely predictable, in a stable price, food and housing environment, is a completely different experience from having no idea whether you will be able to eat next week or if the money will be suddenly taken away because someone saw you walk a metre in a park near your wheelchair and decided you are "fit for work" as a result, even if you can't walk 2m.
Both are experiences of poverty, but very different kinds of poverty.