The Solas arc has been planned out since Origins. Patrick Weekes has said multiple times Solas is crit-path first, romance second. He said this to Solas-mancer. The End. Yes. Things can change - like they may add some more significant scenes to address the romance and you may get some special cinematics because he's has a noisy fanbase. However, I think you over-estimate his popularity. His romance is not the most popular. I think Cass and Cullen both beat him out. He's just so restricted as a romance. So to put an inordinate amount of resources to make a small part of the fanbase happy where it actually may weaken the overall story they want to tell...I don't see it. Again, that's not to say he won't live, but I do believe what you get is what everyone will get. He's too major. Too important. If he's alive, he's alive for everybody because that would mean he would reappear again in a big way. If he's dead. He's dead for everybody and his death will be important and impactful not just to the Inquisitor but to the world.
Love Interests don't deserve to live anymore than anyone else in a story. All that means is that a certain section of the fanbase may be sadder. If it makes story sense to kill one or it is more interesting then they should die. If it's for cheap sads, then I agree. They shouldn't. Also, I could see any of the love interests dying. Cassandra especially. And I WOULD CRY but I could see it being beautiful.
Just because his path has been established does not, by any means, imply he is doomed. One can say Alistair's path was also pre-established. Or Anders'. Heck, one could consider Anders a retcon, since he kind of did have the option of a death path originally (in Origins). But there were still choices we could make for a happier outcome, both in DA:O and in DA:2. And it just wouldn't be in the spirit of Dragon Age if they didn't give us that: the ability to influence the finer details of an outcome, even if the outcome (in essence) has the same broader consequences on the realm as any other path. So nope, I'm not buying that!
And on a side note: my theory as to one reason why LI's do deserve the option to live more than any one else in the story? Because they get to interact with us. Even more than friendlies. Yes, Solas leaving wasn't something we could interfere with. But I don't believe something as major as a death would happen without the protagonist having some sway over it.
A happy ending would be great...but not at the cost of screwing up the greater series narrative, a plotline that has been planned since Origins. Bioware has been careful to make the choices impactful, but not plot-breaking in order to not create continuity errors.
Definitely not 
I'm confident we will have a great tale told to us. But like I mentioned above, it isn't assuredly damned to end in tears.
Can I join you in the Team Optimism flag waving?

I try to be optimistic but all the negativity on Solas' fate (on here and on Tumblr) keeps pulling me back to the sads.
Heck that Dragon Age confession of not wanting Solasmancers to have a happy ending (damn you, confessor!) got me sooooo mad I just can't even... Grrr... 
But yeah, we don't know for sure his endgame is just tragedy and tears, even though people seem to think that is what's going to happen. So I'll join you in refusing to believe that all we'll get is sadness until we see it on-screen with our own eyes.
Because I am stubborn as hell.
I am stubborn with you!!! I don't even think we're being stubborn, to be honest. Without firm evidence, which we won't get until it's release, there is no basis to be negative over. And if anyone doesn't see room for a positive outcome, they're just plain pessimistic -- or hold a lot of love for tragedy! Don't get me wrong, a good tragedy is fantastic and cathartic. But I foretell that there will be more than one way to crack this egg (pun intended).
-waves gigantic, billowing flag of optimism-
That doesn't mean they won't add in some scenes to satisfy romancers, but if Solas was going to do a bad thing or die, he's still going to do the bad thing or die. Especially since he's clearly story critical, I don't imagine them changing much of anything within the basics. Particularly not for a not-too-popular romance, which is likely what Solas is outside of us fans that like to talk about Dragon Age on the internet.
But I trust them to at least make his story interesting and worthy of his character no matter what! Patrick Weekes will see to it, especially now that he's the lead writer.
I don't think the plot will lead to Solas' death at all, to be honest. But if it does go down that path, I still maintain that it will have branches of severity -- and not necessarily result in his death. And that, in turn, could lead to a romance friendly ending.
And to the last bit, I agree 100%! Whatever happens, I feel the writing team will deliver us a great story.