Someone did a study that suggested the network effect is logarithmic in nature, but if that’s true it means something with 95% market share is 3 times “better” for having everyone there. Not 20 or two orders of magnitude better but not nothing.
Please could you link to the study if you can find it. The definition of "better" seems quite critical here, as well as whether it's specific to code hosting websites. A messaging platform is pretty useless if I'm the only user, but Github would still be useful to me without anyone else having an account.
And the audience you want are users who will never, ever pay to get access. If GitHub starts throttling free users, they throttle the audience you're talking about far, far harder than you.
The audience will go away to somewhere you aren't. Then what?
No one needs github for git itself, nor even for web front-end to git, nor even for hosted/managed someone else's problem web front-end to git, nor even all of that for free.
More realistically, MAU or something is a metric/OKR. It’s easy to imagine why a business wouldn’t want to cap that.