I think you're drastically underestimating the amount of code Github is powered by and how freaking long any type of refactor/rewrite would take. We're talking about years.
Does anyone have link to any interview or article talking about granularity of their architecture?
It's talking about a component of Github backend called Resque (Distrubuted Job Queue)
As long as I see these thing the architecture is highly distributed and it's possibly composed of micro services.
I've been through a number of large rewrites/reworks that took monoliths much like Github (with many many many services behind it) and split them up into modular pieces and it's an insane amount of work that can take years. You simply need very good reasons (including business reasons) to do that.
Moreover, companies at these sizes just have a LOT of code all over the place. Tooling, infra, supporting services, etc... Not to mention it's just not useful to have external contributors for a business product like Github. Doing code reviews, addressing bugs that were introduced, spending time discussing things with contributors takes an incredible amount of time.
Basically if the reason you want Github open sourced (and reworked into some weird architecture you described) is so that people can contribute to fix things and add features....Github could/will just hire more devs to work on that.
Github.com = Github Core + Production Services and Infra
Github Enterprise = Github Core + Services need for Self Hosting
Maybe it's not worthy to proceed based on assumption but if there is something like "Github Core" which is shared codebase between prod and self-hosted, open sourcing the core can be an option?
Then possible path might be isolating least coupled (and small) components of client side code and open source?