I think I answered my own question and this is in the realm of very hypothetical since getting to whatever a "maximum difficulty" looks like is hard to define and it doesn't even really matter since most of the supply is out there.
Discussion
Well … even when all the bitcoin is mined, miners will still be competing for transaction fees. I think one way to look at it is as long as the cost of energy + compute + land … is not actually zero, the mining difficulty will adjust accordingly.