Nobody really knows what goes on inside a marriage except the people who are in it. That being said
I don't see William and Kate ever breaking up, for the reasons that
@Andora and
@airgelaal already cited. The only reason I could see it ever happening is if either or both was deeply, deeply unhappy and they thought it better for the children (William lived through his parents' divorce, but let us also remember that he lived through their unhappy marriage first), or if Kate decides she can't go on with the charade, especially when she becomes queen. If that was the case, I could see her going the way of another Kate - Katie Holmes - and pretty much disappearing from public life. But if I was putting money on it, they'll be together as long as they both shall live.
As for Harry and Meghan, to me the more likely to leave would be Meghan. Harry barely knows what a good marriage is, and as already noted, he gave up a lot and I can't see him going back into the family, he's still too bitter about all of it. I could see a situation where at some point his brother and father welcome him back, but only if he has nowhere else to go. It would be sort of benevolent, like a royal pardon or something. But more than that, I think he's going to stick with Meghan, stubbornly perhaps, not willing to admit he made a mistake and telling himself it's for the children.
To me the more likely would be Meghan. She's already ripped that bandaid off with one divorce plus she's cut off members of her family (with good reason of course) and cut ties with close friends too (notably her former bestie Jessica Mulroney) - in other words, she doesn't seem the type to stick with relationships that aren't working for her. Meghan seems to be the one driving a lot - the initial attempt to capitalize on their title with trademarks and a website (there's no way Harry would have done that on his own), the various media deals, the famous friends, now she's writing children's books (who isn't these days lol) and just got herself a fancy new publicity firm. I don't think anyone will disagree that she's ambitious, again not a bad thing at all, but if people get in her way, including Harry, I could see her moving on. I'm sure she'd always put her kids first, but with both parents being children of divorce and her being especially close to her mother, it's not out of the question to co-parent. For her it's about her personal brand, and I don't see her stopping any time soon in pursuit of whatever she has defined as success for herself.
www.mercurynews.com
when I hear the phrase "you're just jealous" or "they're just jealous," going back as long as I can remember to schoolyard taunts. 
