As far as I know, with 2nd layer technologies, you can pretty much have lower denominations than a satoshi.
Correct. The base unit in Lightning is already a millisatoshi (msat), or 0.001 of a satoshi. So even if one satoshi = $1 (which as pointed out above, would result in the marketcap of bitcoin being around 25 times higher than the amount of broad money in the entire world), you would still be able to divide down to the equivalent of 0.1 of a cent (which is obviously more divisible than fiat). And this is without even mentioning that fees on Lightning are generally a fraction of what they on chain.
lets not get carried away though. we still have a very long way to go before we can even begin to think about splitting bitcoin into units smaller than 1 Satoshi.
so far the smallest regular transaction paying the smallest fee is paying a total of a little less than 200 satoshi which is still 200 times more than the smallest unit which means we have 200 units to go before we reach the base unit.