Making it easier to join means more marginal people, with less attachment to the idea, can join. This is still good if their membership adds value, but they dilute the membership, which means we shouldn’t account for the average new member being signed up now as being equally valuable as the members who joined up in 2010.
but just am concerned that this means our forecasts might overestimate the value of adding another member, causing relative over-investment.
Yup, I agree:
but just am concerned that this means our forecasts might overestimate the value of adding another member, causing relative over-investment.