I agree with @fbudin and @dzenanz.
As a Windows user, I guess the pain is mitigated since the advent of Community Editions of Visual Studio.
From the medical imaging software industry point of view, I’d say that this might be more of a concern for companies that may be selling software based on ITK and who used VS12 or earlier to clear (FDA or CE) their product.
But as said earlier on another thread of the discussion forum, we cannot have a modern toolkit using not-so-modern tools 
The Gearing up to ITK5 announcement already forecasted the support drop for VS12 (2013) as a likely one.
So I’m fine with dealing with the trouble this can cause now to produce a better ITK.
Now, if VS14 (2015) support is patchy and buggy, and if our buildbots are reporting errors, then I’d postpone the use of constexpr. However, it looks like we have only one VS 14 build at dash4win7.kitware.com, which is not reporting issues.
Dropping support for a 2/3 yr-old compiler does not seem desirable to me. So may be further testing is needed?