8 decimal places are already bordering insufficient: there are real world currencies whose unit costs less than 1 satoshi. See https://www.fiatmarketcap.com/

Yeah, but in how many of those currencies are people actually using base units for transactions? I'd wager they're all trading notes denominated with thousands or millions of units for a loaf of bread, like the Zimbabwe trillion dollar note, http://news.bbc.co.uk/2/hi/africa/7832601.stm

Also, there is already a trick on the lightning network to trade sub satoshis without requiring a hardfork, probabilistic payments, https://courses.csail.mit.edu/6.857/2017/project/7.pdf