I agree that a UBI will probably be needed in the future, and that a wealth tax alone won't fund it - but it could be part of it. The way I see it a wealth tax is more like a 'catch all' tax - it's a fallback in case the other taxes and regulations that should be in place to counter-act income inequality fail. Those rules are much more important, but we might as well have the wealth tax too.
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.