> You can say that because it is universal, it should level the playing field just at a different starting point, but you are still creating a situation where even incredibly intelligent people will choose to pursue leisure over labor, in fact, the most intelligent people may be the ones to be more aware of the pointlessness of working if they can survive on UBI.

This doesn't seem believable to me, or at least it isn't the whole story. Pre-20th century it seems like most scientific and mathematical discoveries came from people who were born into wealthy families and were able to pursue whatever interested them without concern for whether or not it would make them money. Presumably there were/are many people who could've contributed greatly if they didn't have to worry about putting food on the table.

> The most intelligent will prefer a system where their superior intelligence provides them with sufficient resources to choose a high-quality mate.

In a scenario where UBI is necessary because AI has supplanted human intelligence, it seems like the only way they could return to such a system is by removing both UBI and AI. Remove just UBI and they're still non-competitive economically against the AIs.