1. The author claims the lower bound is above 0 because "Bitcoin offers people a money storage and transfer system with two key properties: (i) permissionless access and (ii) decentralized database management." That might be true if Bitcoin were the only technology that offered those features, but it isn't. Cash is the most notable competitor, but there are also dozens of other cryptocurrencies which are drop-in replacements for Bitcoin.
2. The number of Bitcoin users has a hard cap because the technology is not scalable. It is just plain impossible for a large number of people to use it. This is not like Facebook, where the system is scalable and user growth is determined almost entirely by user interest.
Also, those graphs make me nervous. Fitting a line to a log-(log-square-root) graph using linear regression sounds awfully fishy to me. Would anyone with a better grasp of the statistics care to chime in?