Oh, and another thing to bear in mind: UBI isn't a leftwing policy, it's a right wing, libertarian policy from the Heritage foundation. The key goals of it are to minimise government, and to harness capitalism's inherent strengths/purpose.
I'd like to point out that what you said about being forced to participate in the market makes it not capitalism is false. You have to participate in the real life market, no man is an island, as it were.
Also I'm not saying there's no risk corruption in this system. I think there's far less than there is in the current capitalist and welfare systems, since UBI is designed to kill it, but we still must be vigilant.