It may arrive one day and leave the next. It may arrive at 2 a.m. and leave at noon, but, absolutely, positively, there will be a bus. Mexican bus drivers are probably the best in the world. I've never seen a drunken or doped up Mexican bus driver. They have families and want to get home to them, just as much as you want to get home to yours.
Most bus plunges happen in other parts of the world, so relax.
Enjoy the view. In very rural areas, its customary for people driving non-commercial vehicles, to stop for people on foot, and offer them a ride. I don't hitchhike, but, in the country side, mountains, or rural areas, I never refuse a ride offer either.