Fixing all the bugs in the v1.29.x line of PTR patches, plus re-adding all the features lost since v1.27a (like ambient sounds) will hopefully allow Blizzard to build a strong reference version of the source code. Ideally, this would be the equivalent of what already exists for the many Quake engines. In order to achieve this imho mandatory step, I believe that XP users should not be left behind, because they too may contribute to it.
After Blizzard has a working, solid, modular and bug-free code base, then they may decide to make the bold switch to 64-bit / DX 11 / VS 2017 / Bnet 2.0 integration, to fulfill their vision of what the game must be. And leave XP and Vista users with the best possible working version ever for their OS.
Such bold and daring technological choices have been done before with other AAA games, like for example Doom 3, or more recently Wolfenstein II: The New Colossus (a.k.a 'Vulkan or die!')