Why everyone seems to be absolutely sure Renesmee will stay with Jacob? OK, I know she will, because it's Stephenie Meyer who's writing this (and her romantic endings are very previsible), but I think it would be more cool if Nessie just decided stay with Nahuel (or even stay alone) and Jacob's impriting shaped into friendship only.
I think the fundamental reason for assuming that Jacob and Nessie will marry is their biological compatibility. Canon makes a big deal of pointing out that Jacob and Nessie both have 24 chromosome pairs, and that Nessie has some of the properties of a werewolf: an unusually high temperature, considering her father is a vampire, and an accelerated growth rate. For a lot of readers, this felt like groundwork for an eventual, “They can reproduce!” reveal.
(An aside: you know what else has 24 chromosome pairs? Potatoes. I’m still convinced that hybrids and wolves are too genetically different, chromosomes aside, to have kids, but I know I’m wrong.)
Personally, I’m with you: I’d much rather have Nessie date Nahuel, or one of his sisters, or a vampire, or some random human. Pairing off a tiny baby with a werewolf guardian who will become her boyfriend is not a fantasy I particularly enjoy.