This to me is the center of the conversation: we're choosing whether we need the instances badly enough that we tolerate some, ahem, bad behavior.

I dispute that. To me, the center of the disagreement is between two different kinds of consistency: On the one hand, there's the consistency with a view of the world that treats One as a special number different from all other numbers. This view is based on the real world where singularities seem rampant. On the other side is consistency with a math-y view of the world that wants to unify as much as possible so we can reduce the number of models, thus, work.

But if you want to treat the cardinality of one specially, do you want to drop Const and Identity, too? Const is closer to tuples than lists are, so why not cut them out as well? But then we had examples in just this conversation where Const and Identity where really useful. What argument is left to remove instances for tuples? If you can get over the 5-second weirdness of Const, why not tuples?