Jim,
1. Whether they might have married someone else (my answer below was yes) doesn't change the fact that they are good together as a couple.
2. Yes, in Austen's world, love ( romantic love that makes us think the other person is a soulmate and the only one to share a lifetime with) wasn't the primary reason to get married. Marriage was the means for the woman to support herself and get financial security preferably for a lifetime; for the man, the means of begetting heirs and continue the family line, secure the wealth of the family for the next generation, and filing the role of mistress of the house/estate. Obviously, a woman was a fool to marry without her future being assured (to a man too poor to keep her in the lifestyle she aimed at), and her family was also a fool to give her to such a man. Hence, Mr. Collins was a suitable marriage prospect for most Merytonian girls and families.
3. True, Elizabeth doesn't choose Darcy out of a circle of suitable suitors ("competition"). I suppose this is where Peter's story prompted this discussion (I don't want to talk about that situation since I dind not read the story and don't want to turn the discussion that way). However, she was not in a hurry and might have afforded to wait if she did not want to marry Darcy, especially at the end of the novel when the marriage of Jane and Bingley assured the family they won't fall into genteel poverty.
4. I find it strange that you think Elizabeth only (or even primarily) married Darcy because he saved Lydia. Yes, Darcy had the means to do it but the point is, he had the intention, the determination to do it for Elizabeth's sake (remember, she thought he was happy not to be connected to the family after their fall from respectability and not to become related to Wickham - it might have been a choice but for the fact he loved her).
