And if we start getting transactions down to that point, it is trivial to extend the protocol.
Actually, the fact is that it is basically impossible to add any more precision. It's possibly if the whole network upgrades, but that's basically creating a new network. It's just as "trivial" as changing the 21 million total into something else.
Considering that we will have several decades of warning of the impending need to switch precisions, I still think it will be trivial to upgrade the entire network.
Changing the generation scheme to give more than 21 million coins would also be technically trivial, but the social effects would destroy the whole system.