Capitalism is a fantastic means for directing economic forces as long as everybody has enough capital to do the voting, as it were. If only a few people hold all the capital, then only a few people make decisions and will often engage in capital strikes rather than risk the chance that their position may be threatened.
I just read Progress and Poverty by Henry George and he makes this point very clearly. I highly recommend giving this book a read if you haven't yet.
Correct, they grow out of having a middle class with purchasing power, who can buy products and services, thus creating jobs for those who provide those things. But the middle class's effective purchasing has been going down for decades, while the wealth at the top has been increasing. So something needs to be done to balance these flows of capital - the current situation is not sustainable.
If the problem is middle class purchasing power, then the solution should be to directly improve middle class purchasing power (via a UBI).
And no, a wealth tax will not come remotely close to funding a UBI.
This feels like a fallacy