What? That doesn't make any sense. A political party can't threaten to take away anything when you stop voting for them. Instead, you simply get the policies of the party you elected in their place. If UBI is popular then there's nothing stopping the opposition from offering it, or even promising to double it. But even if they said "elect us and we'll take away your UBI", then that's just not getting a hamburger because you ordered the pasta.

And what aspect of that arrangement looks like servitude? I don't usually associate servitude with the idea of getting paid a stipend regardless of whether I choose to work. Do you know who has historically enjoyed such a stipend? The Lords and Ladies who had all the servants!

Also, there are plenty of implementations of a grocery subsidy that don't have the state giving money directly to big agro-business. For example where I am, groceries are exempt from sales taxes; this is equivalent to a flat tax plus a specific grocery subsidy applied at point of sale to the consumer. This benefits any food producer, large or small.