Right off the bat I would say that his premise that Bitcoin is secure now is flawed. As we discussed extensively in another thread here, Bitcoin is absolutely vulnerable to a 51% attack and it wouldn't be difficult to a large nation-state player, or possibly even a smaller one. Bitcoin has a systemic security risk, always has, and always will.

He is right that thinning margins for miners could very well increase security risks for Bitcoin ("reducing security" not a correct way to say it).

He's wrong that increasing transaction fees will make any difference in the sustainability of Bitcoin since, at $50 per transaction, the price is already uncompetitive with almost any form of money transfer today. But Bitcoin isn't used as a money transfer mechanism today, it's simply an investment instrument.

In other words, if shares of Berkshire Hathaway cost $1000 to transact in, would people stop buying these $400k shares? Of course not. Almost all buyers of "Bitcoin" these days do so through broker of some kind that isolates end-users and spreads the transaction cost across thousands of end-user requests.