I won't contest that there are advantages to the linux Kernel's workflow, but there are downsides too, and a major one is that it scares off potential contributors.
That said GitHub definitely is far from perfect as well, and has different strengths and weaknesses from email based flows. As do any other options.
But just because there isn't currently anything that is unilaterally better doesn't mean things can't be improved. There is clearly a problem with onboarding new developers to the linux workflow. That should be acknowledged, and a solution sought. That solution doesn't have to be switching to GitHub or similar. Maybe there just needs to be better documentation on how to set up the necessary tools, that is oriented towards developers used to the Github process. Maybe there needs to be better tooling. Maybe the mailing lists need to be organized better, or have the mailing list automatically add metadata in a standard, machine-readable format to emails. Etc.
it sounds the opposite of optimized to me. Unless we're optimizing for something other than developer experience and efficiency?
Any change introduces anomalies and those cause all sorts of hell.
It's almost like there are more important goals wrt software development.
obligatory reminder about breaking someone's workflow https://xkcd.com/1172/