I totally agree with you. And to add to your point, the impact of having a dominance in the market of "personal project repos" extends far beyond individual developers paying money on their personal accounts and/or becoming paying customers when they start their own companies.

If you think about mid-sized software companies (~10-100 devs), if 80% of your developers have a strong preference for Github over Gitlab or Bitbucket, then that company is likely going to stick with Github or decide to move to Github eventually. I think this is where Github captures much more than that 12% revenue that they are giving up right now.