I don't really feel that you being a millennial or using vim is relevant.
I have no idea how you would go about getting github to implement whatever functionality is desired and even so would come with the following disadvantages
-- Requiring a chrome extension limiting people to using chrome + a singular website vs any email client under the sun
-- wouldn't work offline
-- Presumably it would be more challenging for sight impaired individuals to use a js heavy website vs their email. They stated they had existing contributors who were so impaired.
-- Since people still presumably do a lot of discussion via email it would mean using at minimum github AND email some of which would refer back and forth to other discussions.
-- Relying on github would require tying free software infrastructure to a single non free software tool. Recalling bitkeeper this didn't go well last time.
-- Hosted github costs additional money.
-- The primary and perhaps only benefit would be making it easier for people to come in off the street and drop patches with minimal effort but this would actually cost effort to manage and the kind of individual who can't or likely wont figure out how to use existing infrastructure may not be an optimal contributor in any case.
Basically your solution involves not only effort to create tools that don't exist, the cooperation of a third party, but the end result would almost certainly be objectively worse in a number of ways.
I feel like you didn't really address any of the things the actual expert on the matter said and just hand waved his entire argument away.