I personally am very happy about this. It will benefit the vim project tremendously.
NeoVim has my full attention; it's a forward thinking project that is doing everything I hoped Bram would do. With that said, it's going to take years before distros start standardizing on neovim, but it will happen.
The GitHub workflow for this - submitting a patch to a project you're not heavily involved with - is very good, and probably a large contributor to the way GitHub is eating everything. No need to subscribe to a mailing list, and no need to get involved with discussions you might not be interested in. But you can easily commit to staying subscribed to your pull request/issue thread.
Of course, that may not suit all changes - but as a default, I think this arrangement a good one.
I've been loosely involved in 2 open source projects which were previously hosted on self-hosted SVN repos.
After a move to Github, not only did the projects pick up new developers, but also gained a bit of visibility.
Github is something unique in my experience... it really is the "place to be" for developers wanting visibility (for any reason). A lot of folks use it as sort of a resume of skill and activeness in the community, and having public projects contributed to under their account serves to bolster that reputation.
How are they going to do that? By selling private repos?
Well, yes. Github makes most of their revenue from Github Enterprise ($5,000+ per year) for large organizations.
Take 10 migrated github repos at random (1) If they did not increase(2) the number of serious(3) contributors, I lose 10$. If they did, you lose 5$.
(1) random sampling: take a repo by using the last digits of a public number (like, say, the nasdaq index). Eliminate repo if "too small" or "not migrated", until you have 10
(2) compare 1 year before github to 1 year after. This gives you three numbers: at start of first year(A), at start of migration(B), at end of migration(C). N1=B/A, N2=C/B. If avg(N1) not > avg(N2), you win
(3) serious means 20 contribs in the year
(further terms will be settled upon if you want to bet, before you accept)
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(btw, I think it might not mean much to VIM that it moved =P)
Also, the metrics you propose are quite meaningless. There are contributions that actually waste core developer's time but are grudgingly accepted either for political reasons or to shut up the proponent.
These kinds of contributions are certain to increase by a migration to GitHub.
Also, fixes seem to be flowing from vim to neovim.
Before Github, the primary means individuals discovered projects was just by stumbling upon random websites... often times ones that hadn't been updated in who-knows-how-long.
Github clearly shows the health of a project by providing at a glance how long ago the most recent commit was, how many people have contributed to the project, how many people are "following" the project, how many contributions via Pull Requests are pending, etc.