I'd be disappointed if every romances ended well no matter what.
If my Cullen leaves my Inquisitor for some reason, I would be sad, but I would reload and make other choices to keep him.
If reloading doesn't help, because BioWare chose to be very cruel with us or I don't WANT my Inquisitor to make the choices necessary to keep him, I'd draw lots of loving Cullen to cheer me up. 
Haha, maybe that's the difference between us--I haven't had one end happily yet, unless you count the possibility that (ME spoilers in case anyone here still hasn't played)
so for me a happy ending would be the refreshing change of pace and a sad ending would be the cliche.

Although of course i hope there's loads of horrible, wrenching drama before reaching that happy ending.
Part of me wants to re-play the last 80% of DAO so that Alistair stays a warden (stupid Hardening quest, happening so early; at that point I thought for sure Loghain would end up King if Alistair didn't, Anora didn't even occur to me), especially since I can no longer headcanon that Amell wound up with Cullen, but (
A) I'm worried we won't see Alistair in DAI unless he's the king and (
B) I totally lose my suspension of disbelief when I do anything a second time, so if I do that in the back of my head I'll still be thinking "no, you aren't Wardens together happy and in love, he's the king and you're a mistress who feels like he's ashamed of you and won't ever really trust him again!" And for that reason I won't be able to go back and reload any major Cullen-romance-killing decisions either, so drama ahoy whether Cullen will date a mage or not. (I still don't believe Bioware will do that to us, though. Ugh.)