First, LLMs learn patterns, not just copy and paste. If they generate verbatim copies of any non-trivial-enough part that would be the subject of a copyright license, yes, it would be copyright infringement. Yet, could anyone give such practical examples? And if so, how do they differ from a software engineer who copies and pastes code?
Second, if the code is hosted anywhere else, there is no guarantee that Copilot (or another model) won't learn from that. The only way to make sure no one and nothing will learn from open-source code is to make it as closed as possible.
Third, for me, the crucial part of open-source code is maintenance. GitHub is there and works well both as a platform for creation (I consider GitHub the most productive social network) and an archive. "No GitHub" (even as a mirror) means that the code is likely to be stored in places less likely to engage collaborators and less likely to last long.