And yes those people who are great and are judged as Outstanding or are working in more difficult roles (e.g. in inner cities with more challenging catchments) should get paid more. Its the same in industry and based on merit. Currently it doesn't matter how hard you work as a teacher until you hit threshold.
This might as well be one of the reasons why some teachers actively choose not to build upon their own knowledge. When I think back of Middle school and High school, I think back of a lot of old people who are so far away from the industry they could never compete in a professional setting. And then there's others that are actively getting certificates, attending additional training programmes and excelling at what they do best.
Without a push forward (or a monetary push forward), it will forever be "oh hell I'll get paid so I'll do my minimum required work and that'll be it" for some of the staff.
In my opinion, a school is a miniature society with minorities, the rich/poor, the dumb/smart, the sociable/introvert, the workers/slackers, etcetera. Having a partially non-qualified teaching staff might say something about a society. Apparently it's sometimes allowed to do whatever job you want without having all the required certificates. In the field I work in (bio industry), you will get in serious
legal issues if someone finds out you're handling laboratory animals without a certificate, or handling radioactive material without the certificates, or even being
inside a higher category lab. I don't see at all why there should be legal exemptions for certain industries. If there is a deficit in teachers, it should be solved by promoting the field to middle-scholars who are in the middle of their career decision, not by lowering legal requirements.