Namur: Yeah, one does have to wonder how did people quit the game back in the DOS days.
Rebooting or powering down somehow seems a bit extreme ;) but definitely a possibility, all things considered.
Speaking as someone who played MM1 during its first release over 20 years ago, yes, we just booted from the game discs & powered down the computer when we were done.
But you have to remember, Personal Computers weren't very "Personal" back then. DOS was hardly a user-friendly Operating System. There was no GUI, everything was on floppies (some games were so "big" they needed to be on 3 floppies that you'd constantly have to switch during the game), there was no hard drive like today (although I was amazed when I read in Compute Magazine about how to set apart a portion of my 512K memory as a "Ram Disk", which functioned like today's Hard Drive), and there was no Internet available to download anything from.
So in a nutshell, we played our games for the PC just like we play our games now on Consoles...put the disc in, turn the computer on, shut it off when you're done playing. We weren't "always connected to the Internet," so we really only turned the computer on/off when we wanted to play a game.