These 291 people haven't pledged dollar values. They've pledged percentage incomes.

Ah; I thought that people were pledging a percent and then listing their estimated income. Looking at an old email from gwwc I see in their discussion of pledge calculations:

We lack income data for some people, which we left out, causing an underestimate. Estimate of lifetime earnings by each member for him/herself * percentage pledged by that member, summing over all members who submitted estimates.

which makes me think they are working off of self-reported estimated lifetime incomes. Though they might be extrapolating from people who submitted income amounts to the people who didn't.

a linear drop off in pledgers with 50% donating for 40 years

I don't think that's the right way to interpret "pledge". If 100 people pledge to keep smoking and then only 50 end up going through with it, I would still say that you had 100 pledges.

£112.8M over 291 people and 40 years is ~£10K, or an estimated income of £100K at 10%. (Which is the same number you got, which I think means you didn't actually apply your 50% figure.)

An average income of £100K is high, but looking over their members, remembering that their students are Oxford students, and figuring that people who go into banking or something via EtG might give larger percents (while still having more to keep for themselves) I think their numbers are not "optimistic to the point of being incredible".