The real problem is that even for companies that wish to pay more and wait more for secure-by-default can't easily tell the difference.

The only solution I can come up with is some form of certification or paid code review from a third party. I know that at least for Windows prior to 7 Microsoft actually allowed some parties to come in and check the code/checksum on an air-gaped computer. We somehow moved to "trust more" in the last decade, and now we can trust nobody

I've yet to see any form of certification or paid code review I'd be willing to bet critical infrastructure on. And working in safety critical software, that's not for lack of trying. Good review is usually harder than building a working system and the asymmetry of offense and defense applies to anything you miss.