A valid interpretation... or was
I obviously don't think there's any hope anymore but they really did a bad job "wrapping up the inquisitor's story," that ambiguous line was just one of many examples. 
It doesn't matter way she revealed.
The way she revealed what?
Solas wanting to kill everyone doesn't make the Inquisitor some essential figure, and it doesn't make Solas a personal rival or antagonist. That's all your perception of your character. That's the point.
Solas revealed himself and his plans only to the inquisitor, the inquisitor vows to stop or redeem Solas regardless of player input. The inquisitor is alive no matter what. Disband or keep the inquisition, it's all based around stopping Solas, every inquisitor ends up in the basement plotting against Solas. This was not my perception, this is fact. Regardless of if this fell flat with you or not, Solas was a companion that turned on you, manipulated you from the start, and was the cause of the breach and everyone who died because of it and it was clearly BioWare's intention to have an emotional moment between Solas and the inquisitor in the scene where you confront him. The inquisitor isn't essential in that they're special and the only one who can stop him (that concept is stupid and I hated it in DA:I), but having an antagonist and a protagonist with a history and knowledge of each other (at the very least) makes for a better story for a lot of people than "random good guy kills random evil guy." Why wouldn't the inquisitor be the one to stop Solas? Why would they pass the buck on to some random level 1 goat farmer? I think a lot of you just want a knew character and don't care about what would make a stronger narrative or what makes sense. But hey, you got what you wanted.
You're missing the analogy because you can't divorce your view of the character from the actual narrative relationship between the two.
Yeah, no. Again, only DR wardens have any reason to chase Morrigan (and she even tells you the baby won't be some kind of demon or monster). Solas blatantly said he's going to destroy the world and every inquisitor vowed to stop him. Your analogy doesn't work.
As to WH, I'm shocked you'd cite that as a conclusion given that it's entirely the same thing as Trespasser. It has the same cliffhanger non-conclusion. If you count that one as an ending then Trespasser is certainly an ending.
Honestly I think BioWare throwing a bunch of mysteries and plotlines around and then either not finishing them or finishing them in an unsatisfactory way is a huge weakness but again: optional mysterious baby =/= your former comrade chops off your arm, says he will destroy the world, you're the only witness, you vow to stop him, you start working towards stopping him.
DA:O had a solid conclusion, DA:I had a solid conclusion but Trespasser tore that open and basically gives your inquisitor a new mission.