And it's a slippery slope. Should, for instance, a chemist have the credential? It's easy to say, "No". Until you get another Bhopal.
Basically you would start getting more and more fields demanding credentials for liability purposes over time. Some would be entirely justified, like chemist and biologist. Some would be tenuous in the extreme, like c# or javascript monkey.
1) An external event, like a war, that stress test the credentials and to force selection based on outcomes. 2) Some sort of monetary benefit for employers, like extended internships for high schoolers. Assuming it's cheaper/more effective for an employer to train their workforce from scratch than pay the full salary of a recent grad. 3) A new field, where credentials haven't been established yet.
There are obv caveats to all of these. And they don't address the question of what a formal education is supposed to accomplish. At some point, it was supposed to be to train "better citizens". And that shouldn't be dictated by employers, imo. But nowadays it seems like the purpose is to get a job and survive.