If you throw in a century of economic growth then 9 digits could be plausible as well as follows:
Suppose Bitcoin absorbs 10% of M2 and the M2 growth rate of roughly 4% persists for a century.
That would imply 1.04^100 = 50.5x growth over 100 years.
That would bring the 10% estimate from 3m per Bitcoin to 151.5m per Bitcoin 100 years from now.
These kind of valuations are highly flawed. If bitcoin ever absorbed even a fraction of "M2" growth it would be largely as a pure unit of account, not a store of value. In other words you'd (by definition of M2) be talking about BTC denominated bank deposit accounts and money market funds and all kinds of other fractional reserve derivatives.
Use of bitcoin as a unit of account is something hodlers tend to ignore. If bitcoin ever became a currency we would not be exchanging actual bitcoins, but bitcoin denominated credit just as we use arbitrary units of credit today. So you can't just divide random incumbent money supply figures by 21 million to get a price for a future BTC. Since it's limited in supply it's an asset and will always be an asset.
If you denominate, say, UK GDP in bitcoin then it's around 0.3 Trillion BTC. The GDP can be 0.3 Trillion BTC even though there are not that many bitcoins in existence.
Obviously, if UK GDP were to be valued in Bitcoin, then Bitcoin would rise in price in order to be able to accommodate UK money supply in the allotted number of Bitcoin units.
might satisfy yourself with Bitcoin denominated credits. I think
already understand the folly of such acceptance.