> If you guys that use GitLab or GitHub or alike would have to switch to pure git - what is the one thing you would miss most?

Github & Gitlab do tons of things which pure Git does not:

    - issue-tracking
    - wikis / documentation
    - release and package-management
    - CI
    - a free of cost (and effort) "public facing website" for your project by default
    - notifications to collaborators

I'm sure there's more I've missed which I would remember in a heart-beat if I was forced to go plain git and were suddenly without, but those are the most obvious ones I can come up with without even thinking.