Han didn't go back for Threepio, he went back for Leia.
He opened the door and kept the ramp down just long enough for him to get inside.
If he had kept the door shut and legged it nothing different would have happened other than Threepio being blasted into outer darkness and Han knowing a little less about how his ship worked.
He would however have to explain to Luke why he left one of the few links with his dead step-parents.
He will also have to answer to Artoo (and I wouldn't want to get on the wrong side of a sentient Swiss army knife.
I get accused of imagining plot points but there is at no point any mention or hint of secret Rebel fighter factories or any private company secretly supplying the Rebels with brand new ships.
If you want to believe that. It's your interpretation of the story which I can't see fitting as well as those ships being beat up ancient relics from Star Wars gone by.
Yes Star Wars is fantasy it can have magic but it should still follow the rules of common sense.
If the rebels are wearing camouflage they clearly don't want to be seen.
If they are creeping around giving each other hand gestures they don't want to be heard.
So bringing a brightly coloured, reflective, loud, panicky butlerdroid to a forest moon makes no sense at all (it pays off but they had no way of knowing that).
It breaks the fantasy if the writers don't take their imagined world seriously.
It also breaks it when references to the Wizard Of Oz, Tarzan and Stepin Fetchit are shoe-horned into the mix.
I don't want to be reminded I'm in a cinema down the road when I should be in a galaxy far, far away.