> my "UBI" would be a couple of million dollars.

There's a difference between what your human brain would become accustomed to (which you'd be right, it'd scale up and up forever) versus what would allow the base level of health and opportunity. As in, not having to worry about eating the next day.

And because you're right that human brains strive for more wealth, UBI should grant you the opportunity to pursue it without fear of failure.