The reason we have antitrust regulations is because the world ended up in a very bad place without them.
This: "GitHub shouldn't be allowed to deny me service because I want to use it" isn't a fair paraphrasing of my argument. I don't think it's likely that github would ban me specifically.
My argument is Microsoft (and Google, Facebook, etc.) shouldn't be allowed to cancel/bankrupt/ostracize competitors, critics, political opponents, or others they don't like. That means we should all be able to access those platforms under equal RAND terms.
If Microsoft is allowed to play dirty, the major impact on me is indirect, in that we will have fewer checks-and-balances in society (people and organizations will be afraid to criticise them), less competition, less innovation, more political corruption, etc.
Antitrust hasn't kept up with technology. In this case, though, we developed perfectly good mechanisms (and learned what happens without them) a century ago.