These guys fall into the assumption that the blockchain can exist without a healthy Bitcoin which is nonsense. If the blockchain is a success, it means Bitcoin is doing great because is the native currency in which miners get paid to keep the blockchain going. Everyone will be using Bitcoin, even if they don't know they are using Bitcoin, as the Winklevoss brothers pointed out. It will be the underlying mechanism of all future electronic transactions.
a blockchain can exist without a healthy bitcoin. (if we define healthy as one that makes mining profitable)
Miners can have incentives to operate without making a profit on mining. Mining companies that have diversified revenue streams, generating fiat - including hashpower rental, exchange services etc, would have an incentive to continue to operate the network regardless of it being profitable purely from mining.
Bitpay, or coinbase for example would have an interest in operating a transaction confirmation service (mining) in order to maintain their business models so long as the potential cost of operating the miners was lower than the potential profit from the service that the blockchain enabled.
Don't assume that mining has to be a source of profit for miners - the blockchain enables other services to be offered to customers - so long as those services can generate revenue someone will have an incentive to maintain the blockchain - even at a loss from mining alone.
"a blockchain can exist without a healthy bitcoin. (if we define healthy as one that makes mining profitable)"
I think this reasoning is fundamentally flawed. It's clear that every blockchain needs a token, in this case Bitcoin is the strongest one.
The more infrastructures and systems running under the BTC blockchain, the more money invested in mining, because these big companies depend on a strong network. The more companies depend on Bitcoin, the higher the price due that fact alone, it becomes even more valuable regardless mainstream adoption or not. And miners will always get rewards in the native currency (Bitcoin) before anything else. The more miners, higher difficulty.. everything is interrelated. There's no scape from a higher Bitcoin price if Bitcoin is a success in any shape or form, if this correlation is not seen then the price is temporarily manipulated.