The requirements for LDAP should not (just) be dictated by how many users you have, but how many 'things' (systems, services, etc) they need to authenticate to. This is why I have AD in my home lab (where I'm the only user). As others have said, organizations of all shapes and sizes need central authentication.
LDAP support is not difficult to add to a FOSS project with a properly designed authentication framework. Limiting it as a paid-only feature unfortunately will put it out of the reach for small businesses and other non-"large enterprise" organizations. There are still plenty of features you can keep in the "Enterprise Offerings" (true SSO, Compliance and Security Auditing, Server-Side Encryption, paid support, the list goes on).
While MM (from what I've seen/tried) is clearly one of the best platforms out there for FOSS on-premise team colab; I cant in good conscience recommend it to any environment that I'll have to support. We would have effectively doubled the number of user accounts we'd have to manage.
I hope MM will reconsider their stance on LDAP. I created this forum account just to post on this thread. While MM looks truly amazing, LDAP/CentralAuth integration is the one feature that is a go/no-go for anything my team deploys.