I think it is reasonable to object to "from outside". marcan is literally a seasoned kernel maintainer. I don't think he needs to be explained what the scale of kernel dev is.
This is a meaningless non-defense of the status quo.
For example, what if there was a forge using open software for each subsystem? You could then integrate whatever tooling you'd like with it.
Bram (rip) made an email based workflow on top of GitHub for vim e.g.. It can be done.