The company that employed the developer ultimately holds the accountability in the marketplace. The employed developer maintains (or loses) their job because of their accountability to their code (or, at least, they should). There's an economic incentive for all parties involved.
In the free/open-source world the incentives aren't economic, but they're still there.
At the start of this you said: "For 99.999% of people, it is literally kthxbye on all code they execute on all their devices."
I think that's inaccurate. The vast majority of code running on "all their devices" is code made by employees of companies being held accountable through traditional industry methods, or free.open source projects where reputational integrity was at stake. Those developers have been held accountable, for some value of accountable.
Maybe there's less value in human accountability than I think there is. Only time will tell. That's a different conversation.
The code running "for 99.999% of people" is not "literally kthxbye" LLM-generated code without someone behind it holding accountability. Maybe it will be in the future, but it's not now.
The company that employed the developer ultimately holds the accountability in the marketplace. The employed developer maintains (or loses) their job because of their accountability to their code (or, at least, they should). There's an economic incentive for all parties involved.
In the free/open-source world the incentives aren't economic, but they're still there.