My son and I completed all of the content with 2 characters.
Certainly the game was probably more fun in coop, but you have been lucky enough to have someone to coop with. I had plenty of friends try out the demo and say "Thanks but no thanks".
Then, if you play online, there is only usually 2100 people playing in all of the US and any other region that may use the US Server at PRIME TIME weekend nights. Then if you hit the party button you can seldom find a party, hence no coop.