One of the reasons this unavoidable deadline was invented, is that the alternative is that one company (or all of them) can simply decide to ignore the vuln report, and then the vulnerability will stay forever undisclosed and forever out there in the wild. And prisoner's dilemma suggests that most companies would chose "do nothing" in this scenario: they don't have to do anything, and if the vuln stays undisclosed, it probably won't be exploited anyhow. Win-win!