I think a lot of the disappointment stems from the fact that this story differs from the previous in some key ways.
DAO: Rag tag bunch of allies who set out to stop a massively larger force. Even after you accumulate your forces you are still outnumbered. I think this is said after you reach the main gate, where you choose the companions you take with you to the final battle. Something along the lines of they can't hold the gate for long.
DA2: rag tag bunch of friends who end up fighting far superior forces. Quanri curb stomp the city. Templars have a huge number.
So when you do beat them its an OH YEAH moment.
In contrast DAI has a different formula.
The beginning starts similar, rag tag group set out on solving a problem where they don't know a lot of what they are up against. The glimpse you get of the future in both the mage and templar quests gives you an idea of what is going to happen if you don't stop it. Then you are attacked at Haven, this is a surprise attack and Cory has you at a disadvantage.
Things shift when you get to Skyhold. You know what the enemy is planning. The rest of the majority of game is about you undercutting his plans while building your forces. This is a vast organization, you are not alone and part of the point is to grow this organization. By the time you get to the end he has nothing left. So when people say he is throwing a tantrum, he is. You have taken everything from him, all he has left is a final last stand with the only thing left to him. The orb and his dragon.
To why Cory doesn't seem to be doing anything before or to counter your actions well I have a few theories.
1. Skyhold is a fortress that would be really hard to lay siege to. He just doesn't have the forces to assault the keep. If he had a demon army then maybe, but you stop that.
2. Does almost nothing to stop you from stopping his plans: He is arrogant. He doesn't think you are going to be any threat or know what he is doing. After all he doesn't know you know what his plans are. If he had succeed in his plans he likely would have curb stomped you at some point. But you do know. By the time he realizes you are a threat you are too strong to assault directly.
3. Solas says it best. To acknowledge you is to undermine his own belief in his godhood. He can't acknowledge you because to do that is to diminish his own power. This arrogance is his ultimate downfall in someways.
Sure would it have been nice to have more moments where you fear you might lose some companions or something. But I'm okay with not having it. But the end battle is fine in my mind considering the set up.