A couple of linked questions really. Firstly, let's say BTC is worth $50k US, that means to purchase your groceries might be 0.002BTC. A sandwich might be 0.0000598. Has a system already been proposed to have better units? With 1 US cent equating to 0.000002BTC could we ever have precision issues?
Secondly, with a hard limit of 21 million BTC and a world population of 8bn, we're talking each person on average could own around .0025BTC. Which at current market cap, is about $100 US. By contrast there's approx $40tn US in regular cash, about 40-60X the market cap of BTC if I didn't make a silly mistake (and depending the value the day you read this).
Doesn't this imply that for BTC to be a real-world currency, it's already large value must end up more like $2m US per coin. At which point your $2.99 sandwich is going to cost 0.000001495 and we're needing 9 decimal places for accurate accounting to the equivalent of one US cent today.
As a newcomer to this world, how does BTC plan to work around these issues to be a practical everyday currency? Is it a trivial thing, or actually a problem?