I think the point here is that we can propose different scenarios all day long; every couple is different. From my own experience, break ups only delay the inevitable - unless there is a compelling reason to begin with. I'm talking money, children, substance abuse issues, which may or may not be resolved. Of course, none of those things apply to William and Kate.
IMO, there has been an underlying current of dissatisfaction from William over the years with Kate. He seems to get "claustrophobic", which is a word touted by the press when they broke up toward the end of their time at St Andrews. The words he allegedly shouted in public back in 2007: "I'm free" spring to mind too. Then, you have the reports of Kate "mothering" William ... perhaps she figuratively smothers him too.
Most men worth a penny get tired of that treatment fairly quickly, in terms of a permanent relationship. Of course, the cads lap it up, because they are only using their partner, and I would never accuse William of being so deceitful ... and, men are known to be "hunters" - they like the proverbial "thrill of the chase". Anyone too easy and available is a turn-off eventually - ie when they get tired of having their egos stroked and having their way. From my own experience, from what I've been told and read, and that of every attractive man I've known or know, this is 100% true. It happens eventually, if not right away.
Sometimes, the simplest explanations are the most accurate: if it looks like a duck and quacks like a duck ... it is a duck. William will go kicking and screaming to the altar to marry Kate, IMO. It's not that I dislike Kate personally (I don't know her), it's just that I don't think she's suitable for William. She does not seem to complement him, either. For example, he needs less of the coddling, and more of a stern hand, IMO.
Kate, on the other hand, might be happier marrying a very wealthy man/aristo who can take care of her. A man who will be happy to show off such a pretty, devoted wife and mother to his children. A woman who will not embarrass or otherwise cause trouble for him, and stay loyal. A man who will want his wife to look after the household and not work outside of the house. A man who covets a wife who will take care of him, spoil and reassure him. I see all these qualities in Kate.
But, for a woman who allegedly did not hold ambition for a career, Kate is grasping for a position that requires hard work and stamina, with no option to be a "stay-at-home" wife and mother. I just do not see this being Kate's "bag" at all. I believe in her own mind she wants it, but the girl is too protected, IMO. In many ways, she may be a "lamb to the slaughter" just as Diana was.
I believe that had Diana's marriage been really loving and committed, we would have seen a lot less of her, charity-wise. I believe that Diana thrived because she put her energies into her charities, partly - but not completely - to compensate for her personal unhappiness.