Anyone who watches the show knows the characters were made for each other in the writers’ room, and every episode made it clear that they belonged together, even when it looked like it wasn’t going to happen. Their personalities, their senses of humor, their interests, their values, etc., all aligned, to the point where it was goddamn frustrating when they didn’t just hook up—and beyond that, get married already. When they finally did, it felt more rewarding to me than other will-they-or-won’t-they couples on TV, because Jim and Pam felt more relatable and realistic than, say, your Ross and Rachel.
I don't disagree with this, but it seems to define "success" as "not breaking up" -- which is obviously part of it, and the specific question being discussed, but I think the more interesting question is whether the couples in question will last and be good for each other. Henry and Casey on Party Down, who owe a lot of their characterization to Jim and Pam (and Dawn and Tim from the British Office), clearly believe in each other's goals, and actively, at times selflessly, try to help them along. They start ready to give up. Henry's ditched acting, and Casey's debating moving to Vermont with her husband and giving up on stand-up comedy. But the show ends (spoiler, I guess, though it's not a show that's easily spoiled) with Henry in a waiting room for an audition, not just for himself, but to prove to Casey he thinks she can make it too. They pull each other off the brink, and keep each other pushing.
Jim and Pam, however, concede defeat. Jim's whole appeal for Pam is that, unlike her fiancé Roy, he believes she can escape. He wants her to go to art school, and not just be stuck as a receptionist her whole life. And yet now, they're both still at Dunder Mifflin, and with a baby and a house, they're not going anywhere. The saddest moments of the show are the ones that drive home how they've given up things that once meant the world to them in order to be together. For Jim, the most notable scene was in "Local Ad", when Pam discovers his Second Life account, wherein he's a guitar-playing sports writer in Philadelphia. For Pam, it was in "Business Trip", after she dropped out of art school, drove back to the Dunder Mifflin parking lot, and tried, half-heartedly, to convince Jim that she didn't even like graphic design:
Now, maybe this isn't just a problem with the two of them. Maybe nothing would have gotten Jim writing a column on the Phillies for the Inquirer, or Pam working as an artist. But the end result is fairly tragic, and their love for each other, by this point in the series, is the only thing they have left. It's something, but it's hard for me to view that as those crazy kids having made it.
P.S. Just to clarify, obviously not getting everything you want professionally is not tragic, nor is making sacrifices for the people you love. But the show is premised on Jim and Pam hating their jobs, and bonding largely over that hatred and their desire to escape, and now they're more entrenched than ever. Particularly for a sitcom, that's a pretty bleak ending.