IBM didn't put much thought into safety. You could blow up the early IBM's if you turned on the monitor (screen) before the CPU box.
Gates noted Ctrl-Alt-Del should have been one button, not three [1]. David Bradley, the inventor of the trifecta, did make it deliberately difficult to reboot, however, it was also originally an Easter Egg which made it to production [2].
[1] http://www.theverge.com/2013/9/26/4772680/bill-gates-admits-... [2] https://en.wikipedia.org/wiki/Control-Alt-Delete#History
I'm not sure a single key would have been a good idea. "The reboot key" just sounds like a mistake waiting to happen. I've seen enough stories on the 'net of laptops with power buttons in terrible places on the keyboard to get a glimpse.
The mistake was using it for the Windows NT screen lock/unlock. Changing the "reboot your computer" sequence into the "start using my computer" sequence is a rather non-sensicle (ignoring implementation) choice.
It already did happen: I've used keyboards with shutdown and reboot keys, and yes, it's a terrible idea.
My alma mater used to run programming competitions in a lab where the workstations had the reset button exactly at knee height. This wasn't a problem normally, when you're sitting under the desk next to the computer; but when you have three or four people gathered around the screen, kicking the reset button was a definite possibility. Eventually taping the cover of a calculator over this button became part of our regular routine.
The power switch on the IBM PCs were way at the back so that people couldn't unintentionally reset the computer. The same thinking went into Ctrl-Alt-Del, which was a combination that people wouldn't accidentally hit.
So having a system where F7 would reboot the entire system was pretty dumb, even in the early 80s.