The classic UI has been preserved throughout generations of Windows development, and it was always kept as an optional alternative to the "newer" or "modern" theme. So don't really know what's the point with Win3.1 as it's already been 20 years of it's gradual evolution! While start menu did not "evolve" to the "void" of it. They simply removed a useful button without a real convincing alternative and it isn't optional.
But your point about batteries is absolutely invalid IMHO. It is okay when you migrate to newer environments or solutions sacrificing backward compatibility as you offer more features and extended capabilities, kinda like the migration we had to go through from 16bit to 32bit and later 64bit programs support, though even in that case, backward compatibility is still existent EVEN NOW.
But the removal of the "Start" button was just like the stupid move MS has done with Vista when they removed the "New Folder" button in explorer. They had to bring it back in Win7, and honestly it was one of the very first and praised features I've noticed in the system!
You're removing a feature that is useful and being used by many, without any gain in performance, capabilities or features in return. And it's not even optional.
Oh! And just about the Windows 3.1 thing, if you check the "Compatibility" tab in your application's properties window, you'll find that you have compatibility modes for all systems dating back to Windows 95, plus 256 colours, and 640x480 resolution modes. And believe it or not, I still have video games from the late 80s and early 90s that are running quite smoothly and perfectly with these compatibility modes in an almost native-feeling 16bit VM. So YES, MS CAN KEEP THE START BUTTON, it's not as if they don't know how to do it! They just wanna force Metro as the only option.