It's pretty simple, Visual Studio Code, the .NET Core runtime for Os X/Linux, ASP.NET 5 and TypeSript are only useful to do Web dev. With Azure they provide the best cloud to run those web app. So they don't lose any VS customer that do software dev, not lose much web dev VS customer on windows because the full VS will always be better and they provide tools for every web developer on other platforms in their area of expertise : development with code completion tools. It's a pretty smart strategy I think.