Github has done nothing of the sort - they're popular, but being popular is not the same as having a monopoly.
> The likelihood of your software being discovered, used or contributed to goes to zero as soon as you host it anywhere but GitHub.
Untrue. It's less likely, because again Github is more popular, but not zero, because popularity isn't monopoly. Active projects do exist elsewhere, people still use projects hosted on Sourceforge FFS.
Unless Github can forcibly prevent competitors from arising by leveraging their control over resources or a legal advantage granted them by the government, they don't have a monopoly.