All further subdivision will have to be external to the protocol, because everything in the bitcoin protocol is nothing but an integer, to soft fork in more subdivision we'd have to allocate a separate space in like the timestamp or something and mark it as a decimal. But it runs into its own problem because if the decimals add up to being more than a single satoshi combining a few txns, but the old network only sees the integers, it'll appear invalid because there are more sats than there should be in the UTXO. This would cause a hard fork.
in other words: The bitcoin network actually only sees satoshis, there is no 100,000,000 unit at the protocol level, and that's way harder to change than people think.
