Jules was right for not accepting Rue’s proposal
Up until her death, Rue wanted to put responsibility for her sobriety on someone else. She quite literally says it: “I want to live for something bigger than myself.” In the end, the fate of being Rue’s wife would not make any difference for Jules: she’s would still be a function in someone’s life rather than her own person. This would never help Rue to actually become sober, just like it never helped in high school when Jules was Rue’s savior, the manic pixie dream girl archetype without a life for herself. Jules may not fully realize what she got herself into with Ellis, but life with Rue was by no means a solution for it.