How much money would World of Warcraft gold be worth when that game eventually shuts down?
How much money will any amount of Bitcoin be worth when the Bitcoin network frays apart and ceases to function?
It's true that is the price goes low enough mining would decrease significantly, but after difficulty adjustment it would still exist to a nonzero extent. Bitcoin can absolutely still function as a system even if 95% or even 99% of all miners end up quitting it over the next few years.
Bitcoin isn't. It's merely distributed and it depends on having a singular, healthy, functioning consensus network. Where that network splits you suddenly have more than one blockchain, as is the case with Bitcoin Gold, and Bitcoin Cash.
It also depends on having a healthy balance of mining power in that network so that no one entity can take control and start to dictate terms.
If all that's left is a single miner then four other miners can easily take charge of that network and effectively destroy it.
> Bitcoin can absolutely still function....
The blockchain component of Bitcoin can technically function, sure, there's no problem there, but as a unit of value, as a unit of exchange, it's functionally useless. You can't trust it, you can't use it for anything.
There is, by design, one blockchain. When that assumption fails, the network has to work to achieve consensus, and if that can't be achieved it splits. That can be fine with things like IRC networks or peer-to-peer exchanges, you can always do a messy reconciliation later, but with something that's supposed to represent a medium of exchange that's a problem.