That's as may be. But I think that if you're going to reject a view because it doesn't sufficiently define good or bad thresholds, you should be able to do the same.
It's not like it doesn't exist. There's presumably some outcome scenario where you would have to admit it'd been a net negative on your chosen terms, and thus a bad deal. One poster on here has it approximately defined in availability of calories per day. I think that's much more bleak than I'd draw the line at, but it's a start.
You can certainly label the whole thing 'not my circus, not my monkeys', but if you (a general you) are going to even attempt to integrate a position on Brexit into a political outlook that's predominantly concerned with the welfare of others, you should probably have an idea of what set of effects on that welfare makes it acceptable or not. The lack of that is one of the things that pisses me off the most about U75 discussion of Brexit as a positive or neutral.