> I don't know what possible source gave you the idea that divisibility == unlimited, but sadly, it's wrong.
It would require a hard fork, but it is technically possible to increase the divisibility of the base satoshi unit. And it should be an uncontroversial change if needed, since it will not require any redistribution of money.
With that said, it doesn't seem likely that the value of a bitcoin will increase in the near future to the point where we will actually need more than 21 quadrillion units of precision.