Those percentage of SWE working on critical infrastructure should be licensed and legally liable, yes. You shouldn't need a license to build a CRUD app, just like nobody needs a license to build a treehouse in their backyard. You should need a license to work on a car's braking system (https://safetyresearch.net/toyota-unintended-acceleration-an...) or radiation therapy machines (https://en.wikipedia.org/wiki/Therac-25), just like you need a license to engineer a highway bridge.
I agree completely — the lack of proper professional licensing around working on what has become critical infrastructure astounds me. But even more so is the pushback I feel from within the field! It isn’t that the pushback isn’t understandable; its more that its disappointing.
Interesting that some of the most critical software is open source with code contributed by people not even paid to write it.