Your point about it going to everybody is a good one, and is the way social security managed to get past the conservative politicians in its day.
My question revolves around how society decides on the size of the allotment, balancing the temptation to demand endless increases in UBI against productivity.
How do we make it reasonably valuable while preventing it from becoming a political bribe to buy votes? In a comment below someone offered the suggestion that one can accept UBI or vote but not both. That would lead to serious, and IMHO, destructive divisions.
In conversations with like-minded coworkers, we thought about dividing a certain fraction of GDP, offset by various costs of government operations, split evenly among all citizens (including their children). In our thought experiments, we figured it might be enough to ensure that changing that fraction would require something like 90% consensus in the voting population (I cannot imagine the number ever decreasing). We were optimistic that level of consensus would be so difficult to achieve that it would offset the temptation to raise UBI excessively.