This would be a question better answered by your negotiations committee. Because an enormous amount of economic research goes into developing those pay scales and how they arrive at the 12 year maximums, which they have decided is what your worth for the majority of your career.

So why 12 years? Its a happy medium. If you wanted a 3 year pay scale to achieve that same max pay, youd be locked up in negotiation/mediation/etc with management for years, and if you wanted a 25 year scale, youd be giving away a lot to the company. The 12 year payscale is what collective bargaining has arrived at as an industry standard. It incentivizes loyalty, but does not force you to have to wait forever to max the pay scale. I understand you feel like you should keep earning more than the 12 year max, it makes sense to think so. But that was never the intent. The max is the max, and inflation should be accounted for every time the contract is re-negotiated. Its not an easy or worth while task to compress the pay scale once its set.