Joss Whedon himself said that on AtS he tried to use Cordelia to redo Buffy's story (shallow SoCal teen who gets super powers & depth) since BtVS the tv series was a year after Buffy became the Slayer so she already gained that. Cordelia's arc and Cangel are LITERALLY a reboot of Buffy & Bangel.
Lol, I must have missed him saying that, but that explains a lot. Basically he saw that people really liked Becoming 1 & 2 and thought he'd just re-create the magic but flip it so Angel had to kill his loved one turned evil? I've called Joss Whedon a hack before, and it seems I'll do it again. I love the shows obviously, but stuff like this is why I cringe when I hear him called a "genius".
Personally, I think that Cangel could've become a real thing, if the showrunner and writers had taken the above mentioned whole doomed to be alone / happiness clause as seriously as they did on BtVS. I feel like they just ignored it (maybe thinking the background was too complex for non-Buffy watchers) but that means they end up with a pair that makes no sense and both act incredibly OOC because of it as Angel's situation has not changed.
Cordelia is the
one member of the AI team who's seen the reality of Angelus. She does not take that lightly, "Oh I'll kill you dead" she threatens (albeit playfully) in season 1 when they talk about it, earlier in the episode -when she thought maybe he was Angelus again - she showed a real readiness to kill him. They should've had a discussion about that in later seasons if this romance was to have a chance, because Cordelia saw the whole Bangel thing and how it ended. Instead they just make up that when she was higher being Cordelia saw everything he did as Angelus… ehm, ok? That's weird and doesn't change the situation either way, but of course, by then the ship had kind of passed already.
Angel already had the experience of a failed relationship with a mortal girl, he knows he has nothing to offer someone like Cordelia, and if he loves her - why would he want less for her than for Buffy? Why would he want to risk going through the heartbreak and risks of that kind of relationship again? They should've dug deeper into his character about why he thought it
could work with Cordelia, or why he didn't care that it probably wouldn't work out, instead of having characters talking about what a true love match they are. Even if they were, it doesn't change a thing from what we've already seen Angel experience and go through, why was he ready to go through it again? What had changed in him to make him think it was a good idea?
I lacked genuine reflection from both characters as this relationship was made to happen and it made no sense as they were both extraordinarily aware of the risks and downsides. But all of that was flippantly shrugged off for training montages, gala dresses, and pseudoparenting.
Characters can grow and change, that's kind of what's great about watching long running shows, I mean just look at Wesley. But Wesley's change is organic, you can see what's happening and understand his development. But you can't just have characters, without any reflection, go "oh this is me now". That's how they ruined both Cordelia and the chances of Cangel for me, because they glossed over all the dark and interesting stuff that needed to be worked through and just skipped to the "they're in love now! Bad idea or not!"
Clearly the happiness clause was, and is, a terrible idea beyond the Angelus reveal of Buffy season 2. It just doesn't work in the long term and the writers didn't know what to do with it or what to do with Angel's romantic life because of it. Because being doomed to solitude is just bad TV. It's not until season 5 that they get Wesley to talk about it and point out that it is unlikely that Angel will achieve perfect happiness with another woman. That's five years into the show! If they'd been serious about Cangel they should've brought that up around season 2-3, imo, though, I suppose they didn't because it would've pointed out that he was unlikely to be perfectly happy with Cordy. Still, ignoring it didn't make things better.