I like happy endings which is why I'm still quietly rooting for one for Solas and Lavellan. I don't expect one of course but I'd like to see an option for a happy-ish ending. That said, I just liked the character of Solas a lot more than Cullen which is why I enjoy the Solas romance more despite the lack of a happy ending.
I also don't really see how Cullen is much like Alistair. Apart from their both being templars and sharing some vague physical appearance similarities, personality-wise they're not all that alike.
I don't have any real preference for or against happy endings. I like them well enough when they feel earned, but all too often happy endings are too... perfect. Like the universe just decides to align and solve all the characters' problems. But then I'm not the biggest fan of the grimdark, "lol everyone dies" type of endings either. Bittersweet endings are the best, in my book. You lose something, you gain something, life goes on.
I'll take whatever PW deigns to give us, as far as Solavellan goes. Happy, sad, whatever - so long as it feels earned.
As for Alistair and Cullen - well, both are goody-two-shoes warrior types, former Templars, wary of mages and especially blood magic, obsessed with duty, somewhat shy in matters of love, have that cliche Disney style romance (Complete with cliche Disney style ending with Cullen.) and serve as blatant Mr Fanservice.
But Alistair makes it work for me, partially because he's got way more personality, and partially because his romance is a beautifully painful subversion of the classic fairy tale plot. (Well unless you play as Cousland, but I found Cousland to be the most boring Origin so....) Cullen, on the other hand, feels like Alistair with all his humor and character sucked out, and his rather clever storyline replaced with some lackluster detox plot.
I was excited for Cullen's inclusion in Inquisition because I thought they'd expand on his character. If you ask me, they didn't. It's the same, superficial side character from DA2 and Origins, just with a more complex storyline.
The biggest hope for some semi-happy ending with Solas is that his writer PW is fond of the character himself and is now lead writer. I wouldn't get your hopes up for a totally happy ending but saving him from himself would count for something where his romance is concerned. I'm more interested how they deal with a male Lavellan who was his friend but has vowed to stop him at all costs because his priority is saving the world. In my case it is not that he wants to kill Solas and would spare him if he could, but that is not his main focus; Solas' welfare is secondary to that of the fate of the world. It is part of the reason I hated that choice at the end of Trespasser and then it being recorded in the Keep because I can see my Lavellan having words put into their mouth (rather has Hawke did) which do not actually match how their viewed he matter.
Even the words they gave you in game were ambiguous. Choosing to redeem Solas was given the dialogue: "I will prove you wrong." In what way? If you prove to him that he is a monster for wanting to destroy this world and thus he admits his error and repents of it, that is redeeming him. If you simply show him that his actions are unnecessary or stop him from completing it, then he hasn't accepted his previous plan was immoral and therefore he is not "redeemed", merely convinced that the alternative is better from a practical viewpoint or is unable to fulfil his plan. His outlook is still the same. His response seems to suggest the latter rather than the former.
You know, I'm like 98% sure there are extenuating (And probably somewhat redeeming) circumstances to Solas' decision to tear down the Veil. Cole's comments after you break up, plus his own blatant refusal to explain his reasoning, shows me that there's more here than "I think immortality is neat and will kill literally everyone to have it." He's trying to save something or someone - not just an idea, not restoration of something that's gone and can stay gone. Someone somewhere's "masked in a mirror," and if Solas straight up abandoned his plans and just had elf babies with Lavellan, they'd probably be doomed to suffer for eternity.
It may not go so far as justifying Solas' plans. But the alternative of simply giving up and going home isn't really morally tenable either. No matter what he does, he's going to be the villain to someone.