For the sake of achieving your goals but not making it painful for those of us who maintain an open source fork: Please consider adding a build-time setting/flag for this so one can set an env var during build to change the hard-coded 10000 limit to something higher or simple have a boolean flag to disable it.
That way you have the 10000 users limit default in your official builds, and maybe even in default Open Source builds (assuming you want this to be enabled by default by distro packages who build from source, since that may be a valid source of future customers after all). But by adding a build flag to allow the maintainer to consciously - yet still easily and without the annoyance of git merge/rebase conflicts - opt-out from the user limit you make their life easier. And I think this is also beneficial for you because some of those people who run such large open-source instances may actually give you valuable feedback!
(PS: I think you know who I am and which instance I run @it33_mattermost considering we briefly met mid-november - just to indicate that the ahove comment is actually from someone who maintains a large instance and not from some random person in the interwebs ;)).