Who's to say your cofounder is going to be as committed as you are when times get hard? When it really comes down to it, you can't trust anyone to be as committed as you to your company except yourself.
It happens. Just because it happens sometimes doesn't mean it always happens. Nor does it mean that cofounders are worthless.
I am not saying being a solo founder isn't viable but I also don't accept your cowboy belief system as absolute.