The Dumat altar in Legacy summons demons, but that doesn't prove Dumat is still alive. It just means demons are responding instead. And considering that Corypheus is doing what he does in DA:I because he believes all the gods are false, and he's taking it very badly, I think that means he hasn't been in contact with anything that identifies itself as Dumat for a very long time. Old Gods have their souls destroyed when a Grey Warden kills them, it's the only way to stop them from regenerating. The only known exception being Urthemiel, provided the Dark Ritual was performed.
So if Old God souls are keys, they're already missing most of them. I do subscribe to the idea that the Old Gods are the Forgotten Ones though, the same type of beings as the Creators, just in dragon form. Which is a reason I believe Solas is upset with the idea of killing them.
It could mean that, but with everything that's happened, I don't see Bioware dropping these little hints and it going nowhere. Especially the text about the old gods being pillars. That's new I'm pretty sure. And they gave us a text that proves that the old gods call out to the blight, and the blight isn't what detects them.
In the western approach, there's a note next to a hole in the ground leading to the deep roads. The wardens were searching for old gods, and when they got close to one, its song was so loud, it shook the earth and caused the ground to collapse. That proves they're actively sending signals. In other words, there has to be an end game here, something this is leading up to. It makes more sense that Solas would be upset about them killing the old gods if killing them was the key to releasing them and the veil truly. Otherwise, he'd just be upset because they're kin, and that seems kind of lame and underwhelming for that sort of reaction.
He was very very upset, and it seems like there's more to it than that for me. They were basically undoing everything he sacrificed for, making it pointless. Also, just because the grey wardens think they destroyed the soul, doesn't mean they necessarily did. You can't really destroy spirits. Perhaps gods can reform as themselves eventually.