Ah yes, the old "Carrot and Stick" philosophy of improving your product.
People bitch about Apple that way too -- nice new features of one product that only work with the latest release of some other product offering, so that you have to upgrade everything to get a feature from one part of the system.
The technique usually works, I grant that.
OTOH, there are fewer and fewer outfits that are still total "Microsoft shops" where it's absolutely essential that everything be the latest and greatest from Redmond. If Server 2016 has features that were added primarily to have something to wave under the noses of the Admins and Execs, and say, "All you have to do to use this feature is upgrade all your clients to Windows 10", there are fewer outfits that will fall for it.
The NT Server line is very mature. Server is a solid product, as solid as it's gonna get, IMO. All that has changed lately (for my money anyway) is the GUI, which adopted Win8's unfortunate Metro look, much to my dislike on a server OS. Other than that the changes have been just window dressing (so to speak).
So if they try to use 2016 features as an upgrade lever, well, best of luck...
There is much more to this server upgrade than meets the eye, and much more I think than we've seen in some past upgrades. Take a look at Nano Server as an example, and consider the kind of bottom-up refactoring it took to make that work.
On the client side, I think the biggest factor is going to be WMF5, and how much of that will back-port to W7 and how soon that will be available.
On the server side, the major change in the GUI is a shift in philosophy. They spent most of their effort on getting rid of it.