Github is indeed not a monopoly as far as a fitted out office building is not a monopoly. It does have an extremely high exit cost. And it's not reasonable for the owner of an office building to refuse access for your staff for saying something horrible about them.

Offices (and SaaS products) are dangling on a thin thread. It doesn't take a lot to lose customers. Lots of them. Especially when working from home (hosted gitlab) is a reasonable option. Most offices near me are converted to housing. You can't convert github to housing so it has even less tangible value if it pisses users off which starts a chain reaction of risk.

It's also bloody unreliable and in the forefront of people's attention for risk already.

Okay then, let's see you try to get a job by telling the hiring manager, "by the way, GitHub has perma-banned me so if you hire me, you'll have to change your entire organization's usage from GitHub to something else."

Let us know how that goes for you.