The maths is beyond me,or at least doesn't interest me,but as an active GM playing against all sorts of opposition in a range of different types of events in the UK,from GM tournaments,International opens,strong team events and weaker team events,to strong weekend opens and a few ordinary weekend opens, I think I can imput something towards this debate.
I have only recently become active again on the UK scene,and have been allocated an adjusted grade of 229,down from 232.
It looks to me,judging by the 100+ games I have played in the last 12 months,that Roger's assessment is entirely accurate.For example,I expect to and do score significantly better than 75% against players around 204 or 207,and that is now - before lower graded players have received their bounties from the god of grading! Perhaps I am not the best example,because I am playing catch up to some extent after my period of relative inactivity.I think my new grade will be around 235 or 6,but Roger is entirely correct that these free points dished out lower down will simply filter up to us and we will indeed be 250+ or so within a few years.
By the way Roger,I think Mark Hebden has been far stronger than keene and Hartston for decades(if we forget about his current dip in form,which I'm sure is only temporary),and I don't think he was ever that much weaker than Miles and Nunn.When all were active together Mark scored reasonably well against these two,and wasn't hugely out FIDE rated by them.
Without realising it Mark will have been continuously improving ever since he began playing chess,and will continue to do so until age/health/motivation etc have their say.I say ''without realising it'' because we all improve together,with time,across the whole spectrum of chess strengths,so this improvement won't show in our results,unless we improve faster than the average;but it does show glaringly when we look at the relatively poor chess we played eg a decade ago when we had the same grade/rating.Because of this continuous rise in standards I think that by objective criteria Mark is probably stronger now than Nunn and Miles were a couple of decades ago.