Secondly, at some point, calculations become onerous. Like, if btc is $100k/coin, then normal, every day transactions will need to be in 0.000001 coins. Which is an incredibly annoying unit to work with, it's trivially easy to miss an order of magnitude, i.e., 0.0000001 at a glance, is that a dime, a dollar, or a ten?

Accidentally give a $100 instead of $10 once, and you'll be annoyed, do it twice and you're done using btc.