Good answer. If the employer is paying the employee to do a role, and that employee is doing nothing but fulfilling the role, the contract of employment has been met. There is an inherent imbalance of power in almost all employer-employee relationships, so chastising the employee for only doing the requirements of the job is unfair.
Since it's already on the onus of the employer to judge the performance of the employee, if they are unable to recognize that the employee is a high performer and has worked himself or herself out of the current role, that responsibility lies on the employer and the employer alone
Under the OP's current situation, if he/she is doing nothing but meeting the requirements of employment, he/she should expect nothing in the way of extra compensation but cost of living adjustments. In the same way, if that's the only requirements the employer sets, there should be no expectation to deliver anything further than what they have asked.
In a pure capitalist perspective, employment is nothing but an agreement between the employee and employer. If the employee meets those agreements, they are satisfying the market. Expecting anything more from the employee is not being a pure capitalist. You cannot have your cake and eat it too as an employer. You either subscribe to the idea of the market dictating supply and demand or you don't. If you find that the employee is gaming the system just like you are, there shouldn't be any judgement or hard feelings.