However, I'm very skeptical about the idea that GitHub is going to somehow become an Azure-only walled garden. It makes little sense business-wise. The entire point of buying GitHub was to acquire an audience that they are aware is not necessarily interested in Azure or their stack. Forcing it on them will only cause them to leave (there is plenty of competition in the code hosting space now), which will in turn reduce revenue. I give MS enough credit to know that the only way they succeed in this space is to provide value, not vendor lock-in. This is true now, and it will be even more true in 5 years when other big companies inevitably start following Microsoft into this space and competing for market share.
If they wanted to just sell services to people already using MS products, they already had VSTS for that.