Speculation on the potential scalability of this approach seems absurd given that membership numbers have been successfully used by organisations of every scale for centuries.
The concern is not with the scale of the organization, but with the number of organizations a user is a member of, which has exploded since website accounts appeared.
It really hasn’t. The number of organisations that wish to track people for marketing purposes has exploded, with all the unnecessary account creation that goes with it. That is not membership, and these are not organisations worth your engineering expertise.
Well, before the Internet I would never have needed an account in a club race management system; I would probably just have used a bunch of unwieldy papers.
While I don't disagree that there's a lot of useless account creation, I'm still member of at least magnitude more of useful online accounts that I or my parents ever were offline.
The concern is not with the scale of the organization, but with the number of organizations a user is a member of, which has exploded since website accounts appeared.