I'm having deja vu, like I've already mentioned 'courtly' love or something. The Inquisitor will never truly be with Cassandra again, to do so is practically thought of as a crime. It's not a healthy situation for a person to be in. He should not let himself get led on, he should drop the false conciliation and move on with his life. If Cassandra loves him then she should make the situation clear, and let him go, to do otherwise is a debilitating form of selfishness.
I can see where you are coming from and as much as I like the divine romance I have to admit that with the limited content provided by the DLC it does paint a depressing picture for the Inquisitor. In fact if it weren't for the extra thought I put into it I'll probably would not feel this good about it. To some degree I can agree with you. Cassandra cannot expect the Inquisitor to be happy with this arrangement just because she is willing to go through it. Eternally loving the Inquisitor from a distance and expecting him to do the same is not a sign of selfless love, like you said is the opposite. This expectations will only lead to frustration, jealousy and resentment. BUT... knowing Cassandra's personality and what makes her such an amazing character makes it easy for me to imagine a conversation between the two where Cassandra tell the Inquisitor that he should find someone else that could offer him what she cannot and insist that the path she chose was for her alone and not to robe the Inquisitor from a happy live. She puts duty first but ONLY at her own expense. So it makes sense to me that she would encourage the Inquisitor to fall in love again with someone else since she wants her Love to be happy. With that in mind and being the hopeless romantic that I am I have no trouble letting my Inquisitor CHOOSE to be by her side, despite Cassandra's best efforts of making him let her go. Yes, I realize that makes the Inquisitor sound incredibly needy but again this does not need to be the case. In this scenario my Inquisitor would not be staying for the slight chance that she might change her mind or for the opportunity of stealing a moment of weakness. That would be awful and like you said super unhealthy for both. So once again in my mind the Inquisitor doesn't choose to stay because he can't live without her, he stays because even though he can let her go and move on he doesn't WANT to. So he decides to stay by her side as whatever she needs him to be, if friends is all they can be he'll be the best of friends. And he will love her without expecting her to give him love in return. If both Cassandra and Inquisitor accept their love in this form (which of course I believe they can) then their feeling will be ultimately selfless and positive, thus allowing them to find true happiness and joy in their companionship.
But then again, that's just me. None of it can be consider truly valid since Bioware didn't even bother putting a last cutscene between the two. So you have a point Hendrix, just base of the DLC it does comes off as a relationship doom to end badly. In fact, in comparison Morrigan would appear much more selfless in DAO than Cassandra in DAI Trespasser. Now THAT is a scary thought xDDDD