> if the cable gets frayed, you have to replace the entire power supply

This is true for the existing design but it would have been easy to redesign the power supply to avoid it.

Just look at the other end: the wall-plug end has long been replaceable; it slides out so you can put on a different plug or a longer cable. This could be true on the MagSafe side as well -- then any Mac power supply could be used with any generation Mac, just by switching out the MagSafe/USB-C end.