Content warning: this is a rant from a teenager who has strong opinions.
Okay...
However, it holds a monopoly on software.
You don't know what a "monopoly" is.
they could just go “Boop! You’re gone!” and there’s nothing I could do about it other than move forges.
Yeah, nothing you could do about it, other than moving to one of the many other git hosts. Monopoly!
And then after listing off a whole bunch of alternative git hosts...
Centralization is not bad by itself but it’s bad when there’s no other option. There just needs to be ways to contribute to code without having to use Github.
You have plenty of ways to do that, and you know that because you just listed them. Github is not a monopoly.
Also, I don't see the concept of open source mentioned at any point in this rant.