This is an interesting question. We don't really know what will happen long term. One possibility, as you point out, is that black markets will always outpay any other market. Another possibility is that the ethical hacker community will become so large and strong that they will find all those same vulnerabilities and deliver them to the system owners before the black market gets to build exploits and use them for nefarious purposes. It takes just one ethical hacker who finds a critical 0day to deliver it to a service like HackerOne, and the market for that vuln is over. Although asymmetry is usually in the favor of the criminal actor, in this case it is in the favor of ethical behavior. One ethical hacker can put an end to the sale of a 0day on the black market.