Questions are how new worlds are designed. :)
I first came up with this idea before I had really gained a full understanding of the Bitcoin Standard, and I do still have concerns about building fully onto Bitcoin with no buffer. Just like with inflation constantly causing prices to go up, I'm a bit concerned with the idea of prices using Bitcoin will continue to go down over time. You're right in thinking that people will not want to exchange Bitcoin for goods and services as much since they might be able to get a better price if they wait a bit longer, but if we need to eat we're going to spend what we have. A token design like what I have in my head would be trying to keep prices as steady as possible for as long as possible and my own thought experiments have helped me reach the conclusion that a currency supply that grows in parallel with population (up or down) would be the best way of achieving that.
Just like with anything new, adoption would be slow at first unless coerced by government adoption. And with government comes corruption so we would need to have the system designed locked down as much as possible to prevent that.
The way I envision the system working, it comes from the idea that people spend a percentage of their income on certain things. Housing, food, entertainment, etc. Setting the supply at 100 tokens per person could help us track and record differences in those budget balances between regions across the world in order to identify disruptions and needs.
Tokens/Credits could be generated when a child is born and a birth certificate is filed, and then burned when a death certificate is filed. They could go into interest bearing accounts upon creation that would help to generate an income for the parents, schools and whatever, then the wallet would be their nest egg when they graduate from school. That's just brainstorming right now.
If the objective is to make money, then we don't need UBI, just let people fend for themselves. But if the objective is to build a civilization that takes care of people, we need systems in place that enable that. UBI is one of those.