It's a 32bit game that uses over 2Gb of memory (the 32bit OS limit for each process by default) frequently, due to leaks in the code. So on a 64bit OS you'll never crash but on a 32bit OS you'll run into issues. They could just fix the code though and it could easily be stable on a 32bit OS. Or they could it make a proper 64 bit application which could bring some benefits performance wise.
Well look at the dx12 situation right now and it's hard to say anything good about it. For all games that have it, it doesn't look any better than dx11 and doesn't run faster - it only runs faster on AMD cards because AMD cards have bad dx11 performance (CPU overhead). So the picture isn't quite as pretty as you make it out to be. Maybe dx12 will be interesting in BF1 but I wouldn't even count on it. It's simply too early. Games need to be built on dx12 from scratch for it to do its magic.