In his interview, Patrick Weekes certainly made it sound like he wrote Solas as a tragic character, or at least a character full of sorrow, remorse, and loneliness. Romancing Solas certainly amplifies that, and it really makes DAI a sad, sad story. I'd be disappointed if they end Solas's arc with his death, especially if the inquisitor has to kill him. Honestly, I can't imagine Bioware forcing us to kill a potential love interest. They've given the option in the past, but never forced it on us.
Also, whatever plans they had for that DA2 expansion clearly evolved since they mashed it into DAI, so really I'm not too concerned with anything Gaider said about that in the past.