I think "the theory" is clever but not remotely going to happen. This is a stand alone film and it's completely over ambitious to believe that literally years ago, they were arching characters in a conceptual idea for a film to a film in production, or some phase of it. And yes, kids will still want to get the merchandise even if the characters die. And it still all adds to the Star Wars universe. People need to get out of this mindset that every character in the new Star Wars universe has to tie in somewhere else. The stand alone "Star Wars Stories" are just that, stand alone. And they are going to be littered with characters that will not have significance outside the stand alone films.
I do agree with TC that Disney probably chickened out on Rogue One, but I still think that most of the Rogue One's are going to die in the film. Or, at least appear to have been killed but not confirmed. I really think they lightened up the tone with the re-shoots but just don't see a happy ending for the rebels.
Now, that being said, in the original teaser there was a quick scene of a robed character approaching what looked like a bacta tank. I guess if I were to buy in to this theory, which I don't, maybe that is the last scene of the movie and it's Jyn in the tank and Vader is off to side showing Palps his prize. Really doubt it though.