Another way to calculate is use MV=PQ formula, where M is 2100 trillion satoshi, V is 0.05 e.g. money saved for use every 20 years, and Q is world GDP at 72 trillion USD, then you get a P of 1.45 satoshi per USD, more or less in the same magnitude
That's a funny way to look at it. In principle you're right of course, but....
it would mean that the full world GDP would be bought with the very tiny amount of BTC that is NOT in saving funds. After all, you have the difficulty that on one hand, all the BTC would be in funds which are frozen for 20 years, and on the other hand, all transactions happen in BTC (if you want the world GDP to be bought in BTC, in order for your formula to work).
It means that the world GDP is actually bought each year by a very very tiny fraction of all BTC while most of it remains enclosed in 20-year funds, which makes that the 72 trillion USD pump up the value of the BTC market cap to 1448 trillion.
The danger would of course be that this high market cap would induce people to cash in their BTC, increasing V drastically, and make the value of a BTC drop at the same rate.
Imagine that you have your retirement savings in such a 20-year holding fund. And imagine now that most people decide to empty their funds, and spend all of their BTC in a year's time (they could in principle buy 20 times the world GDP with it !). That will crash the BTC value a 20-fold. You loose your retirement savings a 20-fold.
I wouldn't want to have my savings in something that had its value because 95% of it is not liquid, and the full market is actually carried by the 5% circulating (like maybe right now with BTC !). The moment people decide to put their stash in circulation, the value of it would crash 20-fold !
Money in retirement funds have the slowest move speed thus impact the money supply in the most significant way, other usages like daily trading or monthly spending does not have the same effect on money supply
The problem with the relation between V and T (the holding times) is that V is the harmonic average of the inverse holding times, and not the arithmetic average.
As such, short holding times dominate over long holding times.
If you have 90% of the monetary mass on 20-year hold, and 10% of the monetary mass in a 1-year hold, the average "hold" time (1/V) is NOT 0.9 x 20 + 0.1 x 1 = 18.1 year, but rather: 1/(0.9/20 + 0.1/1) = 6.9 years. That 10% circulating once every year brings down the 20 year period to a 6.9 year period. In other words, those 10% circulating 20 times faster, make the value drop by a factor of about 3.
So you see the huge danger of having the value of a monetary asset be dominated by extremely low V: the slightest fraction that starts circulating faster will make the value drop drastically.