...something like 20-25% of those involved in 80K have changed or substantially rethought their career choice. This implies immediately that 75-80% haven't, and in practise that number will be higher care of the self-reporting. This substantially reduces the likely impact of 80,000 hours as a program. Indeed, it seems to be a near fatal problem for GWWC, in that if the 80,000 hours population is representative of pledges, then most of the GWWC pledges are earning in line with typical post grads, which makes it much harder to raise the mean value of each pledge to £250K as is required.

This doesn't follow. If you were already going to be an Investment Banker, you wouldn't change your goals based on 80k's advice, but you'd still earn more thanthe average graduate.