I do one 100-mile charity ride per year. The fundraising is optional, and the ride costs around £30 to enter whether you raise funds or not - and the past two years I've raised decent sums.
A recent annual charity sportive has changed its approach and is now hounding riders who paid only the basic entry fee to donate more. Some of those riders donated on the day but it seems that the charity doesn't log the donations made at the start/finish. I was told that there were fewer riders this year, (unusually, I didn't even ride out to the route to cheer my friends past this year - I forget why not) which may explain the new hounding, but I suspect they're going to have even fewer riders next year as a result.
Are sponsored charity rides sustainable? Does donor or participant fatigue eventually kill them?
I think there's only one Charity Ride near me which has been going for any length of time: the excellent Sandringham Samaritans Fancy Dress Ride in mid-May, but it's only 14 miles for the long course and I suspect most of the donations come from the participants and spectators, not others sponsoring them to ride. The last few years, there's also been a Mayday Welly Ride accepting funds for the RNLI as it rides along, which seems a bit different, both in the challenge (ride 20 miles of rolling hills in wellies, or 40 there-and-back) and approach.