Re-painting with same color....process opinions?

My car is in the body shop. I had the same discussion with the shop guy. What pushed it over to the "bare metal" side was when I told him I had blocked the car out using rattlecan primer 20 years ago, prior to a previous paint job. He felt we had to get that layer off the car in order to ensure a good job. If it had been original paint he would have just sanded and smoothed as necessary.