Actually you are right and wrong. A frame rate of 20 milliseconds is 50 times a second. PCM frame rate is just the same as its PPM counterpart. If you could see 50 times a second, it is faster than your nerve paths can carry the information from your eyes, process it in your brain and send a correction to your muscles in your fingers to move sticks. You are right that many manufacturers have come out with a faster rame rate for three channel ground radios (14-16 milliseconds) but these are not used in most planes, so the point is meaningless.

The error detection is all in the receiver as you say, but I may disagree with your understanding of it. The PCM receiver ignores bad data packets and uses the values in the previous good data packet. If it gets bad data packets for a long enough amount of time it goes into failsafe.

Since 8-10 channel PCM IS AS FAST as PPM, the real question is, why have PPM at all? I think it has no purpose. Why design PPM systems that have error detection and Failsafe like PCM? Just for the folks that have PPM only transmitter systems.

Sounds like we could eventually give up PPM receivers all together.

Prejudice agaist PCM is merely a mental roadblock.