No I get what you mean, but Solas never said he would be by your side forever. Never promised his loyalty eternally. He simply said he was there to stop this and that was that and when it was over he would not forget what everyone did. I guess when I think of betrayal, I think of an un-hardened Alistair saying he will be with you forever, then because you are not a noble, he dumps you and takes the crown.
Well we're really going to bump heads on this one because I think Alistair's betrayal is a lot more mild than Solas's and not just because Solas's is literally world changing. Alistair didn't want to be king, he showed you on multiple occasions that he didn't like having to lead and he wasn't cut out to make leadership decisions. You chose to make him king against his wishes, for the *greater good, yes, but then Alistair broke up with you for the greater good. He was doing what he thought was necessary just like you did what you though was necessary.
It was very a**holey of him to do it in front of your companions though.
*I hate the greater good -.-
Now when I talk of Solas's betrayal I don't just mean as a romanced Inquisitor, this goes for anyone, even the dislike Inquisitor as much as a stretch that might seem. You were the leader of the Inquisition, he was working under you, something he did so that his goals could be met. You could of been his friend, and even his lover. He made people believe he was someone he was not, he kept things from you, he planted spies in you Inquisition. There are certain unsaid things in any relationship. I imagine an important one is "I will not one day break up with you out of the blue before leaving only to show up years later and reveal my plans to destroy the world... and take your arm." just because your significant other doesn't articulate this to you doesn't mean if he does it it isn't a betrayal. Some things go without being said. The other doesn't even have to be signification for that action to be a betrayal. Any type of radical, contradictory, unexpected action in any type of relationship can be a betrayal of said relationship. You really shouldn't have to ask your partner or friend if they plan to one day commit mass genocide and destroy everything you know and love. You just expect them not to.
Not everything has to be spelt out and spoken. There are implicit social rules about what constitutes friendship, a romantic relationship, etc.

It doesn't matter what he intended. It matters what he did. No one is saying that the betrayal makes him a horrible unredeemable person.
You are reading my mind Eivuwan!
I just think betrayal is not the right word. I would say he mislead and probably out of fear. Perfect proof is when he expects you to recoil in horror at romancing the Dread Wolf and you can shout at him that you wanted him to trust you. His silent reaction basically says he goofed big time.
Betrayal is a harsh word, but Solas's actions were harsh. I get where you're coming from, I love Solas, but I appreciate all sides of his character, even the bad parts which are as much a part of him as the good.
His goal all along was to bring down the veil and destroy this world in order to restore/save the ancient elves. Having to stop Corypheus was just a hiccup in his overall plans. The Hush Whispers quest is not a good comparison because there is a difference between actively killing a bunch of people and causing a reality to never have existed. I am not saying that Solas is evil, but let's not gloss over the severity of what he has done and what he is planning to do.
So much this! Sometimes I feel like people are too willing to do just that. Recognizing his actions for what they are doesn't mean we dislike him and even if his actions are justifiable it does not automatically make them any less terrible.