Just a note here: cooperative multitaking may cause a system to become unresponsive, but it won't cause a system to crash. In both the case of cooperative multitasking and the lack of memory protection, the stability issues were caused by applications rather than the operating system (in virtually every case). As such, it was quite possible to choose applications that did not affect the responsiveness or stability of the system as a whole. Granted, that was virtually impossible to do for web browsers in the case of the classic Mac OS.
It was usually 68k apps running on PPC machines that would cause the more catastrophic crashes, as they were more likely to corrupt the system heap. The classic OS's main stability issues were with extensions, which were mostly fixable with Cassidy-Greene's Conflict Catcher. Reordering the extension loading usually fixes any issues.
I've found that with any PPC-native program, if it becomes unstable and causes the machine to stop responding, force-quitting would usually bring it back from the dead without causing the entire machine to go down. I still tend to reboot after such an event, to clean up memory. But the crashes are few and far- between with PPC- native apps.
