The Mac isn't locked down like iOS, but there's still no expectation that software built for one OS major release will continue to work for the next.
If the Apple fandom wiki is correct, the 68k emulator for PPC was included in all PPC releases, but Rosetta was included in 10.4 and 10.5, optional in 10.6 and unsupported in 10.7; it's scope was more limited than the 68k emulator as well. I expect Rosetta 2 will have a similar limited lifetime.
That's if you have monopoly like control over your entire eco-system...