£85-90k is about the going rate for a national charity of that kind of income and number of employees. Income which comes from a variety of sources, and spending the money on a range of different kind of project.
I haven’t looked at the demographics of the membership, but it’s entirely possible that the senior rate members are largely wealthy enough to afford the extra (there’s another rate for those on means tested benefits). I seem to remember that (nationally, on average) pensioners are now better off than recent graduates (and presumably 20 something non graduates), who get no rate reduction at the moment. In that context, oap discounts are generally harder to justify than they used to be. They could also have the senior rate track state pension age. Whether those members want what cuk wants to be is a whole other thing.