By decoupling income from work, UBI challenges capitalism's theory of value in a way socialism doesn't. Socialism still believes your value is determined by your work. It just wants you to own more of it. UBI says we are all inherently valuable and your time belongs to you alone.
It's been said by @katjakipping that the old left wants control over the means of production while the new left wants control over their own lives. What's more important to you? Control of capital, or control of your life and how you spend your extremely limited hours living it?
This is where a bridge exists between the left and the right and why UBI is forward. On the right, one could say the old right wants to cut taxes while the new right wants individual liberty. Govt should empower individuals as individuals with the freedom of self-determination.
If your only concern is taxes, you've lost sight of the original goal to "form a more perfect Union, establish Justice, insure domestic Tranquility, provide for the common defence, promote the general Welfare, and secure the Blessings of Liberty to ourselves and our Posterity."
