AS there are only 21 million Bitcoins that can be mined in total and currently, around 18.5 million Bitcoins have been mined.in contrast to legal tender of various countries which has no limit of the quantity that can be printed, I believe Bitcoin to be used for transactions purpose,has it's limits, in essence legal tender will still have upper hands for transactions purpose.
I don't really see the problem, especially when you can divide 1 BTC to 0.1, 0.001, 0.00001, or even 1 sats. Since there is no 'inflation', the purchasing power would theoretically be the same (or at least similar), unless we still peg the prices to fiat.
Anyway, even if we do get that problem (where 1 sats is worth more than 1$), we can simply find new solutions with 2nd layer, just to name a few.