Agree with @princessrococo in the comments about an obstacle and a chalenge to Juke. Outside of that context, and I still wouldn't use the word ship inside it, I don't ship them.
Nick works best to me as a) a not-really-so-nice-guy who only notices Julie once she's moving upward (as @1mnobodywhoareyou have talked about in DMs LOL) and wants to be involved with her because of that, and b) as an almost foil to Luke being a ghost.
Like, obviously we don't know what the coming seasons had planned, so this is conjecture. It's important to note that Julie specifically doesn't drag it out, she knows where her feelings lie and she has no intention of wasting Nick's time, but I can imagine that she has not quite yet considered the actual reality of this crush. At least, she didn't on screen beyond a few lines.
She ignores Flynn's warning of Luke being something she can't have, and the reminder that he's dead; she doesn't really care. Her feelings are real regardless, and as far as she knows, he isn't going anywhere, and neither are those feelings.
We also don't know the projected timeline the future seasons might've had. All five could've taken place in the span of a year. Maybe a few months. Maybe two years and then some. But Julie would've aged, and Luke wouldn't have. While she doesn't drag it out, she shuts Nick down, time passes and people change. And the story that comes out of it all simply wouldn't work as well if there isn't some physical reminder that there was, and could still be, another option.
A live one, not a dead one. But I do think it goes beyond that. Nick acts as Luke's foil.
And Nick works best as that theoretical, representative of life option. Like he wouldn't have even had to be a real contender, right, it's not about necessarily choosing Nick over Luke and vice versa (although. lol. juke are soulmates. so.) but an almost choosing between life and death. Nick is alive. Julie can choose to date him, or date anyone else that isn't Luke, because the choice is really that she is dating someone alive, that will grow, and change.
How the show would've handled this, no clue, although boys coming back to live is definitely my favored theory, but a lot of fic at least tends to characterize this dilemma as loving someone now because you can, and because even if Luke might crossover or if Julie grows older and past him, they had the chance to love each other and they took it, and it doesn't matter if it'll end.
(A lot of fic also inevitably ends up rewarding this realization with bringing the boys back to life anyway, but i digress)
Nick needs to be an obstacle, a brief moment of what if even if it's not about Nick, it's what he represents. Maybe even Luke, feeling bad, needs to consider that pursuing Julie is doomed. Or maybe it isn't. Maybe Julie needs to consider her future, and her choices, and understand that it might be doomed. Or maybe it won't be. A question. A choice. A decision.
Good stories need stakes, and they need to give us moments to grow, and there's really no room for Julie, specifically, to grow without understanding the choices she might have and will have to make. And really, the question that comes of it is this:
If she had known that she might lose her mother one day, would that have ever stopped her from loving her?