The "core essence" is to have one password, instead of having 20 or more. It is going to be hard to convince people using it, if it is to complicated to use. Thus LDAP is the only reason we haven't tried mattermost, yet.
I am a little bit scared about of your "enterprise" version, because you describe them as "hundreds or thousands of individuals" within your vision-posting. We are a small company, still LDAP (compatability to AD) is a critical functionality to us. Please do not forget small-sized "enterprises", because we only need a very small subset of business features (in fact your current product + LDAP) + pricing needs to be reasonable (and most offers out there are forcing you to use either cloud for small numbers or are a pure ripoff for small companies). If you have any further question, ask along or contact me directly.