Well even if we assume there's a suitable 8-pin microcontroller which doesn't cost more than the 555, merely loading the firmware onto the microcontroller is go

Well even if we assume there's a suitable 8-pin microcontroller which doesn't cost more than the 555, merely loading the firmware onto the microcontroller is going to add significant cost and complexity to the manufacturing stage. Also the microcontroller would be far more sensitive to power supply inadequacies because its state consists of much more than a capacitor and a flipflop.