And even if the benefits are entirely "inflated away" in aggregate (which is extremely dubious given that UBI produces real economic value in terms of stability), that's just the aggregate effect. To take an extreme example, if the Fed prints 1,000,000 exadollars and distributes them equally, then yeah it's pure inflation, but it also massively redistributes wealth to the point where everyone is practically identical. Hopefully that example demonstrates that the effect isn't zero even if it's just "inflating itself away."
Finally, I object to treating this question in pure economic terms. We shouldn't be overly concerned with optimizing aggregate economic outcomes for society. Those at the top end can more than afford to give up some wealth, and those at the bottom are struggling to a heartbreaking degree. The utility function of holding wealth is not linear and if you're interested in increasing everyone's well-being then that's a fact you have to address.