Makes plenty of sense to me. As a developer, I like git and appreciate the "added value" from github et.al. In practice, CodePlex has long been abandoned for active development, so I would only look there for something old and obscure. As Microsoft, what do they gain from maintaining a product that's been effectively abandoned by end-users? Given that they've (1) given plenty of notice, (2) are providing migration assistance [how effective remains to be seen...] and (3) will be providing a "sensible" archive of the site available I'm not personally unhappy with something that feels like a pretty sensible decision.