The military flew indexed ‘AOA’ which accommodated configuration changes – the doughnut chevron system. In later years indexed systems were produced for civil aircraft, mainly in corporate operations. Both of these systems focussed on safety, providing approach speed / speed margin and guidance for keeping out of trouble, stall, windshear etc.
If the full value of AOA is to be realised then more sophisticated calculation is used, but then the dominate problem in civil operations is the acceptability of a display. A standalone advisory system can be certificated on the basis that it is not misleading. Using AOA as primary the primary ‘speed’ reference would present significant problems in certification, but probably the issues of training and standardisation costs to the operators would prohibit this option. A counter example is the use of HUD, where the costs have been offset against the Cat3 capability.
On a point of standardisation, speed or AOA, I was always impressed by the US Navy system of colour coding ASIs. This provided speed bands for flap (gear) speed and the range of approach speeds; each aircraft type was calibrated / marked accordingly. Thus it didn’t matter which type you were current on – flying the approach in the ‘green’ band provided a significant degree of error proofing.
Civil flight tests (1970’s) experimented with AOA for auto thrust control with little success, but the military IIRC, were more successful – A7.