No matter how you define it, or whether people even agree companies should be obligated to provide certain public services, we are just nowhere near that line yet in this case, net even remotely close. It’s hand-wavy to say it’s important, but this is all brand new, there are only a handful of researchers involved, the critical mass to justify what you’re suggesting does not yet exist, it won’t for some time, and there’s no guarantee it ever will. I’m not sure what you mean by publicly funded trust, but that’s typically quite different from privately funded public services. Assuming that cost is even the reason here, then if someone wants to establish a trust and engage OpenAI, they can.
That said, what if OpenAI shut down codex because it has dangerous possibilities and amoral “researchers” started figuring out how to exploit them? What if it was fundamentally buggy or encouraging misleading research? What if codex was accidentally leaking or distributing export-controlled or other illegal (copyright, etc.) information? I’m explicitly speculating on possibilities, while you’re making unstated assumptions, so entertain the question of whether OpenAI is already doing a public service by shutting it down.