Capitalism is a system supported by government institutions, such as the ability to form private companies and sell shares or bonds. You can have this type of system in parallel with another, even if only a portion of your population participates in the production or consumption side. This is known as mixed economy.
And if the capitalist consumer-driven economy is floundering on lack of demand, where will the taxation revenue to support the UBI come from?
If anyone is making money, you can tax them. Ultimately the government's revenues are going to be limited by the economic output of the nation. If your GDP is crashing, then you will not likely be able to increase taxes to make up for it. The result in that scenarios is austerity. Where UBI recipients get some reduced amount of income based on what can actually be raised.
This scenario is unlikely if your economy is healthy, regardless of what economic system(s) you implement. If your nation produces something, even if it was done by robots, then you have wealth. It's the decision on how to fairly distribute that wealth where capitalism and communism butt heads.
Obviously the property owning class of a plutocracy would want some kind of meritocracy. Of course with the property owning class making the rules on what meritocracy means.
A purely democratic and egalitarian view would be that everyone gets what they need. Which I believe will be necessary as we approach a society with 100% automation. As not everyone will be necessary need to work in order for the nation's economy to be in surplus.
But the distribution of that surplus among people is currently controlled by some unfair factors, like wealth. But also some usually, but not always, fair factors like did the person do any work. Of course if someone is disabled, they shouldn't necessarily be required to directly contribute a nation's economy. But there are ways to contribute indirectly, like part time community volunteer or to the arts. The basic philosophy is for there to be a place for every person to be the best person they can be.