Classic is not really an emulator, because it doesn't pretend to be another type of hardware. It just lets two operating systems run at the same time on the same hardware.
This is why you need OS9 to run Classic apps. Classic doesn't pretend to be OS9; it runs a real OS9. It does a decent job of it too, most of the time.
To be honest, though, I'm not sure how well Myst and Riven run in OS9. Myst probably runs fine, but Riven might not.