> Using email for git scales extremely well. The canonical project, of course, is the Linux kernel. A change is made to the Linux kernel an average of 7 times per hour, constantly.
Tell that to any big team in a big company or even a medium-sized company using a monorepo. The Linux project has a very high bar for entry and it's not an example of a busy repo.
Linux upstream gets 7 commits per hour, but the Linux project is divided into subsystems which are run like independent projects that periodically push their work upstream. The volume of activity that Linux receives is very high.
Can you back up your statements with evidence? I have no reason to believe that email wouldn't work in a large monorepo-like setting. The less-than-10-in-total companies which have crazy Google-tier monorepos may run into issues with this (honestly, though, I doubt it), but they are not the norm.
Most projects are more accessible to beginners, and GitHub makes it much easier for them to do their first contribution to an open source project.