Admittedly I've been away from TFS for a number of years, however, it seems that GitHub Enterprise is a natural fit product wise for revenue stream via licensing thru MSDN to get organizations onboard with GitHub that weren't before. If they were to sunset TFS and provide a migration path, it would end up giving them significant licensing dollars.
The more important point I was trying to make is that orgs that are now on GitHub Enterprise would need to subscribe to MSDN enterprise licenses for a bunch of things that aren't relevant and get locked into TOS that they hadn't been previously.
Regardless of sunsetting TFS/VSTS, I think that Github Enterprise will be only available through a MSDN enterprise/premium license or whatever that is now in 2018.