For businesses, as far back as they need. Consumers don't care, and for consumer grade OS it's fine. Let's say I invest 1 million in machines that use 32-bit software for my professional studio, and it's working perfectly fine. Then Apple decides one day to remove 32-bit support. The vendor is small and can't easily afford to port their software over to 64-bit. And for what reason? It was working fine from the start. It makes me believe that Windows is the only real professional/industry grade OS.
Note that I am primarily a Linux user, but I know people in industries whose entire business relies on the fact that Windows will never brick their old software that has been going for 20 years. And if I was using that software, I would pick the Windows version any day.