In fairness though, Bitcoin's future economic viability is purely theoretical. We're all just hoping it works. Right now, Bitcoin is extremely inflationary, and that's why miners have such a strong interest in securing the network. In the future when the mining subsidy is no longer relevant, the mining economy may look very different, and accordingly so may the security and viability of the network.
He's not wrong when he says the supply limit can't be changed. Consider things like the DAO fork in Ethereum, and then consider that lack of mining incentives could be an existential threat to Bitcoin in the future. If so, then a hard fork that further inflates the 21M coin supply is a real possibility.
We're all just theorizing when you get down to it.......