There's no reason today's macOS couldn't support a Classic environment, like the early releases of OS X. There are a lot of support costs surrounding such an environment, so I don't blame Apple for dropping it.
It supports x86 emulation, for now.
I believe Windows has seen more architectures than Mac OS Classic and OS X combined.
Windows 3rd party software often drops support because Microsoft doesn't support the OS. It could be the desire to use new APIs that aren't included in 7/8 (or soon to be 10), but it's hard to support an operating system as an app vendor that the OS vendor doesn't support.
I always liked VMware's statement that they would support NT4 and above -- like, no you can't.
* I believe Windows has seen more architectures than Mac OS Classic and OS X combined.*
I have never been a Windows user, but I used to keep an eye on it when NT was still the separate business version (pre-Vista) and my NT 4.0 (or was it 3.51?) CD-ROM had x86, MIPS, Alpha and maybe PowerPC support. When things weren’t as clear platform-wise, NT was really a multi-platform system. Since then also x86_64, IA64, ARM64.
Classic ran as suid root and was a big huge security hole on a multiuser OS. (which is probably why they got rid of it as soon as they could.) There are some more contained emulators of course.
I'm also seeing more software lately talking about dropping support for Windows 7 or 8 after a certain release.