Is Github the long terms vision for source code?

This is something I think about as well. Most of all the learning paths for repos and deployments show using GitHub, so it does look like there is a future path for this.

However, at the moment I would say do what is best for your company. If they do change from Azure DevOps to GitHub they will include an ability to migrate. I suspect it would move from Azure DevOps to GitHub instead of the other way round so that might sway your choice.