I am arguing that is one element of their plan. improve retention rates, particularly with their planned bi-annual content drought.
have you considered how they may try to tie-in classic for retail players? special achievements, special mounts, special transmog, etc? if they don't actually put achievements into classic, there may well be an entire achievement subset for classic within retail, with a whole pile of them essentially requiring level 60.
people actually treat achievements like content and do them. I know it sounds crazy but I have seen it too many times to tell myself otherwise. If they add ach. in retail for classic stuff, there are players who WILL put the time in and do them.