If HHH and Stephanie never got together, when do you think HHH would have made the otherwise-inevitable drop from main event status? Do you think he still gets the big push in 2002 after the quad injury? If he still gets that, do you think he nosedives once it became totally obvious the match quality was lacking (and would not return until 2004)? That 2000-2001 run was one of the best ever, but it's hard for me to picture him in that prominent role beyond 2005 if he's not the Boss (or married to the Boss). If Hunter fades following the initial run, who do you think would have risen in his place? Would Kurt Angle have become the next great heel? Jericho? Someone else?
If it's not Lex Luger, it's HHH around here. Thing is, even without banging the boss's daughter, he was still a giant star after the quad injury, and Vince isn't a stupid guy. I'm pretty sure HHH would have been pushed almost as hard, because HHH is awesome at looking out for HHH, and most of the REAL power moves were behind-the-scenes stuff anyway. The roster was so depleted after the departures of Rock and Austin that they pretty much HAD to keep pushing HHH.
Now if HHH had been dating her in this fantasy universe and then BROKE UP with her after cheating on her like he did with Chyna, that's a different thing altogether. I think that then, yes, Kurt Angle becomes the next guy in line, and maybe Jericho gets pushed to the top relatively unfettered.