He's honestly overstayed his welcome on the show, there is no point in keeping him around. Remember that he only showed up in three books, and was only a major character in two!
Theon will be beheaded by Stannis in The Winds of Winter and that'l be that. We got our redemption arc when he saved fArya, no need to keep this shit going.
Nope, it's simple
Maybe Theon doesn't have a clear arc on the show which makes it seem as though they're treading water a bit,but this:
"Remember that he only showed up in three books, and was only a major character in two!" literally makes no sense.
Theon is a main character, he only had PoVs in two books because those are the only two that required him to be a PoV, while Thrones had constant set up from the first chapter on for what his character would come to be and he's the main face of a major house.
Theon wasn't in books 3 and 4 (only talked about in them) because George had no reason to show his torture unlike the show where that was needed to keep him relevant and show what we wouldn't get in his internal monologue.
Jaime was only in two books as a point of view aside from a single PoV in Dance, that doesn't make him less important but people love to throw that ball at Theon.
Theon is one of the biggest movers in the seires, his actions in book 2 is what paved the way for the Bolton takeover, pushed Bran's plot into motion, caused Robb to seek Jeyne for comfort which lead to the Red Wedding,and caused more in regards to the Kingsmoot and beyond.
There's nothing to be dismissed about that, not to mention book 5 content.
The idea that Stannis will execute Theon is laughable, it's not happening, his arc has a long way to go before he gets the sweet release of death (if even that).
There's character development that needs to be done (such as fully excepting the murder of the millers boys which he has yet to do),reunions (people have been theorizing about his reunion with Jon for years), he'll probably play a role in the taking back of Winterfell and has valuable info on Ramsay he's yet to relay to Stannis, probably going to meet back up with Dagmer Cleftjaw who's still holding Tohhrens Square, and so much more.
Bran's just going to do some magic mumnojumbo to do something that ends up needing Theon to stay alive longer, he's not going to be sacrificed, it's laid out so clearly that I can't fathom why anyone has been expecting his death in the show or books.
Edit: not to mention that Theon was recently talked about by George as being a foil to Jon and how his arc is struggling through to be a hero, or Ramsay being specifically created for him because he "needed something to bite Theon in the ass", and I doubt all that was just for the jump off of Winterfell