ok lets take this one step at a time.
you say bitcoin becomes a global currency, so in that case the price should rise up a lot first because of the adoption (simply more demand > higher price)
first of all it will never be "global" nothing is ever global. so lets say the price goes up to $10,000 since so many users like this number then 1 cent is 0.000001
BTC (0.000002
BTC if it goes to $5000)
so it is safe to assume 1 satoshi is worth something eventually.
with that in mind there is not 21 million coins but there are 21e14 (2,100,000,000,000,000) coins available so there is enough coins to go around. (keep in mind there is nothing called decimal point in bitcoin code, for example right now what you see as 0.1
BTC is what your GUI shows you in reality you should see 10000000 instead)
(but what is the fee to send 1 satoshi ?
)
two things i got to say to this:
1) fees will change with price. and it has already changed a lot of times
2) it has been said before and i say it again if 1 satoshi has a value in fiat then bitcoin can be divided into a smaller amount (0.1, 0.01 satoshi)