I do retrofits and I'll just assume the equipment he was using didn't have an analog encoder option or was ignorant of such equipment (likely using some cheap hobby level CNC thing.) I've worked with a few motion control systems such as ACS and Aerotech and they provide inputs for just about every encoder type imaginable. I retrofitted a CNC with 1500W 90V servo motors with analog resolvers using an Aerotech system, zero mechanical upgrades.

I'm going to forget some details, but he was not using hobby level CNC products. I still have a Kollmorgen AKD motion controller that he sent me as a test bench. Most of the old tools were using 0-10V sawtooth analog outputs for motor position and, IIRC, their preferred controllers didn't support that. The initial version of the box I made took that 0-10V and pretended to be a resolver so the AKD could control the motor.

Like I said, I don't remember all his design decisions, but they wanted to retain the original position sensors.