You are mostly right, I had forgotten about fat binaries. However, 10.6+ only supports x86{-32,-64}, and and 10.8+ only supports x86-64 [0]. Having Windows 10 (?) support 4 architectures is a feature, in my opinion. But, I'd still agree fat/universal binaries would be a usability enhancement.
Aside from universal binaries, Apple's approach to 64 bit also provided a better user experience. Microsoft required you to choose either a 32 bit or 64 bit version of Windows, with the 64 bit version breaking driver compatibility. Apple enabled 64 bit apps to run atop a 32 bit kernel, enabling driver compatibility, at a slight perf cost.
Probably these new ARM machines won't support legacy hardware, though that may not be relevant nowadays.
[0] https://en.wikipedia.org/wiki/Universal_binary