That is a pretty Anglo-Centric viewpoint I would say. I know of companies in Japan that are moving onto GH:E with users ranging from 20 -> 300+ of which there are probably tens of thousands similarly sized around the world that are not yet on GH or GH:E.
Github recently created a Japanese subsidiary which is telling.
The growth will be in non tech companies starting to do software, and non software uses of git, but noth of those will be slower perhaps. And maybe Windows shops moving to git too.
Most of the big tech companies already have a presence there, and they've been the go-to for VCS-using developers for a few years now.