Woah, pretty extensive read. One thing I noticed though, is the author seems to assume that blockchain.com wallets is the only one available for Bitcoin users:
Therefore, if 3 million active Bitcoin users represent about 17% of all Bitcoin owners, then the total number of Bitcoin owners around the world is only around 17.6 million.
(This suggests that nearly 50% of all blockchain wallets (accounts) are either lost due to lost keys, abandoned/unused, or can be attributed to multiple account holders).
Other than that, it does cite its (many) sources and should be a good read for people who want to know more about Bitcoin as an investment.