I think the reason that Solas didn't seem to have an answer to the Blight question was "spoilers". Remember he was also evasive if you ask him why this world has to die. His response seemed to be "Good question, but I'm not telling you." May be he didn't want to say too much because then you might sit back and think everything was going to be okay or resign yourself to the fact that he is right and it is the only solution. He wants you to go out looking for alternatives.
If you think about it, things that Solas clearly ought to know about but didn't expand upon:
Titans
The origins of the dwarves
The origins of the elves
The origins of the kossith/qunari
The origins of humans
Who the Old Gods are
The origins of the Blight - honestly are you telling me he has absolutely no idea about this?
The Forgotten Ones - we were left to assume that Evanuris was a catch all term for the whole elven pantheon yet only the Creators really featured.
Who actually killed Mythal
I'm sure you can come up with some other things if you think about it hard enough. All the things he could have talked about in the Rotunda and all I got was the Shadow Goddess, Barindur, some dwarf disaster and a baker in Par Vollen, among other trivialities. Oh and the fact that he had visited Flemeth's Hut.
When I finally caught up with him, I could have happily sat there and chatted with him all day. It's not as though I had any urgent business elsewhere. The Exalted Council can go **** themselves. So just neutralise the anchor and tell me a story. Lots of stories. Go on Solas, you know you love doing that, and if I'm going to come up with a better plan I need some information to go on.