i do not think that steve and nancy would have stayed together for so long if barb had not died that night in season 1:
i think she and steve's relationship would have naturally fizzled out. nancy would have realized that steve was not really her type and that she was more into the idea of the popular boy liking her than actually being with steve.
i also think that things would almost stay the same except barb survives in the upside down and gets rescued with will. so now, she is undoubtedly a member of the party and this puts her and steve in each other's orbit much more than before.
nancy and jon would still get together. i mean, it's basically inevitable. barb and steve go from strangers to frenemies to actual friends to something that lingers on the side of more. the friendship is a lot of banter and light teasing until it transforms into something deeper.
steve calls her barbie. she pretends to hate it. she calls him a litany of different nicknames, but he likes it most when she calls him steven. they study together often. he carries her books, walks her to his car. she always makes a little show of acting annoyed but she waits by her locker for him after basketball practice every day of senior year.
he graduates with decent grades because of barb. hell, he gets into state with her help. she's going to state too. she insists it is fully a coincidence. with nancy and jon so wrapped up in each other, she has the alarming realization that steve harrington might be is her best friend.
when she has nightmares, she knows she can call him even at 3 o'clock in the morning and he'll sneak over and climb through her window. letting steve hold her hand in her childhood bed doesn't seem so scary when she dreams about monsters with petal faces and rows of teeth.
she defends him to her other friends, even her old best friend robin who insists that steve is a terrible guy. she tells robin she can name a 100 ways that steve is an amazing guy.
steve defends her viciously. anyone who so much as looks at her wrong will get an earful or worse. he'd already dropped tommy and carol, but he loses his last few buddies on the team after he hears them making fun of barb after practice.
steve looks at barb and he just thinks she's beautiful, that she's perfect exactly as she is. he loves her red hair, her soft skin, pretty kissable lips. what others call big or fat, he thinks of as curvy, as soft, as lovely. her laugh, her intelligence, her fierce commitment to being true to herself. steve loves everything about her.
so when he hears the team talking shit about what he sees in her, he gets into another fight. he's got a busted lip and a black eye and a startling realization that maybe his feelings for barb aren't as platonic as he thought.
he doesn't just love her as his best friend. he's in love with her.
when robin enters during season 3, i think steve and barb would become steve barb and robin. the terrific trio. robin wants to shake them both because it's so obvious they like each other. they'd finally get together around the events of season 4.
and yeah, robin gets it to an extent. steve is a great guy even if he is a total dingus. not that she gets the appeal personally.
steve makes the first move after he gets jealous watching her interact with eddie. i think eddie's a shit and would totally flirt with barb because 1)she's a pretty girl and 2) he immediately picks up on steve's interest and jealousy. he's a little shit who likes to stir things up and watch the chaos unfold.
steve knows he has a shot when barb also gets jealous over nancy's flirty glances.
barb stays with lucas, max and erica because she thinks an adult should be with them. before parting ways, barb would make some sarcastic comment about steve making sure he doesn't get another concussion. tells him that he better come back.
and he takes the chance. kisses the hell out of her right in front of everyone. says some cheesy line like 'how could i not come back when i know you're here waiting for me barbie'
and then no one dies and they live happily ever after.
