2,5 million/BTC seems a bit unrealistic to me... But still...
Let's assume that in a couple of years, about 20.000.000 exist. Satoshi isn't around anymore, so at least 1.000.000 BTC are lost.
Let's (for the sake of argument) say that most of those oldtime holders decide to sell their holdings, and 15.000.000 BTC are in circulation (this is a big stretch).
The monetary value of those 15.000.000 BTC would be 15.000.000 x 2.500.000 = 37.500.000.000.000
There are 8.000.000.000 people in the world. That's ~4700 USD worth of BTC for each inhabitant of earth.
If you assume only half of the population holds btc, that's still only $9400 worth of BTC/person. This wouldn't make half of the earth's population millionaires i'm afraid...
Let's add the fact that i don't see 2.5 million/BTC exchange rate, and i don't see those longtime holders all selling their stash to get a circulation of 15.000.000...
This tweet from glassnode is interesting:
https://twitter.com/glassnode/status/1344020551942238212They estimate only ~5.000.000 BTC to be in circulation.
This makes the claim you sometimes see that guesses BTC might rise to $500.000 by the end of the year at least plausible:
(5.000.000 x 500.000 ) / 8.000.000.000 = $312 worth of BTC/inhabitant of earth.
Even if only 1% of the population holds btc, it would only be $31.200 worth of BTC/holder.
I see this as overly optimistic, but not completely inpossible.