We wouldn't necessarily have to go back to that. We would be worse off without them [0], but Honda could get even GM's 5.7L V8 engine to pass regulations without a catalytic converter [1] in the mid-seventies. Eventually they couldn't get their CVCC engines to pass emissions without a cat, but we could make it back to the late seventies / early eighties. And that assumes we have made zero progress in engineering and material science over the last four decades, which strikes me as unlikely.

[0] https://www.designnews.com/stub/we-make-it-simple-how-honda-...

[1] https://jalopnik.com/when-honda-gave-gm-one-of-historys-most...