At the time I joined the Society they were pushing for a target of 10,000 members - how are the mighty fallen!?
An analysis of the accounts would probably show that annual income from members (ie not life members) is a relatively small part of the total.
There are several main reasons for the fall off of member commitment - most of which where fire sale responses to pull up the drawbridge to protect the London centralism.
The Society does not exist outside of London and not even for the average Londoner.
Incidentally I was extremely supportive of retaining Cecil Sharp house when its abandonment was being urged - but that was mainly on the basis of accessibility to library etc. London is still the easiest place in England to get to - especially by public transport.