there is a limit! and that is the total of 2,100,000,000,000,000 units
the fact that it is possible to change (make 0.1 possible) doesn't mean it will change.
it will be block size all over again, the fact that we can have 1000 GB blocks doesn't mean we will have them. changing the divisibility to more than 8 decimal places will be a fork (i believe a hard fork since it is not going to be backward compatible) and will require the whole network to agree about it.