This seems like a reasonable choice to make, to me... DX11 is getting on for seven years old now, so if anyone
did need to upgrade their graphics card (the worst case), they could do so for minimal cost.
The first Nvidia graphics cards to support DX11 were the GTX 4xx series, for AMD the HD 5xxx series. These cards are 6+ years old, and anyone running anything older probably isn't getting an amazing ED experience to begin with!
As for Win32... Same deal, except 64-bit CPUs have been common since... what, 2006? I think it's fair to move forwards, especially if we're talking about 0.5% of people who have ever launched ED.
I can definitely believe that dropping these "legacy" builds and code paths would allow FD to simplify things and improve performance, especially going forwards with new features, whatever order those appear in.