Or else, why do buyers want clients?
The hypothesis I like to come back to --- I'm pretty sure it's true --- is that vulnerabilities have value on the black market only if they fit into an existing business model, such that they can be dropped in and immediately be used to make money. People have to already be using some other vulnerability to do the exact same thing, and reliably making money with it.
People on HN like to tell stories about how a master criminal could make money with everything from Facebook CSRFs to serverside RCEs. But none of those kinds of exploits support current ongoing business concerns; they're all one-of-a-kind. Nobody buys a vulnerability speculatively to see if they might make a go of it --- they especially don't do that for a vulnerability that could be extinguished universally in moments by Google or Facebook's security team.
I could see an argument here towards the market price of the information -- breaching a company in the above scenario might get you zilch. Breaking an iOS device of an individual of interest gets you a lot of value.
Circling back though -- you're saying that this model isn't one in use. Do you agree with my real-life counterpoint? If so, why wouldn't there be a market? Or, is the market there and the low payouts from the likes of Zerodium reflect the actual low value of the product (and by extension, business model)?