If you view Github as just a Git (and occasionally static site) hosting service, then there's not lock-in whatsover; you can always move to somewhere like Gitlab or host your own. But the point is: Github isn't just a Git website anymore; it creates a community around it. Right now the reason why people aren't easily moving out of Github is because by moving to somewhere else, they have to risk getting less views, less recognition, and less pull requests for their libraries. Also, if you were a Sponsor in Github and earning $30000 a month and then had disagreements with Github's policies and want to get out, you now have to risk shaving off all your sponsors to switch to a different service like Liberapay. Maybe some of your passionate existing patrons will go towards the extra effort to switch alongside you, but the reality is: most won't.

There were lots of promises and hopes for the patron economy (or I would extend this to call it a "distributed economy"), where people can directly give money as reward for their work while avoiding the traditional hierarchical structure of corporations. However, because of the nature of the current society we live in, the ideal version of this economy would never come to fruition. Think of examples such as Patreon, Youtube, and recently Github; they're an enabler for diverse communities, rich subcultures, and innovative ideas, but the users still have to live under the guise of huge capitalistic forces. It seems that the distributed economy still has to live under the current technocratic system (where huge tech corporations have much higher leverage than small companies or non-profit organizations). To see this relationship between users and corporations as either symbiotic or exploitative is up to your choice, but I think the status-quo will stay for quite some time.