You could recast Bran and 99.9% of viewers wouldn't notice or care. You could not even pick up his storyline and most viewers wouldn't even care, some wouldn't notice. If Jon, Dany, or Tyrion just stopped being in the show tomorrow with no explanation, the average viewer would notice. I don't think that's the case for Bran.
Your plot armor argument is interesting but I don't think it really holds true. Main character doesn't necessarily equal character that won't be killed off. Main characters are the characters the plot revolves around. Game of Thrones has three main plot threads: the wall, king's landing, dany and the east. Each of these stories is a bildungsroman for our main characters.
There are other notable stories being told (stannis, arya, theon, bran, littlefinger, tyrells), and some of those (littlefinger, bran) may elevate to central plot one day, but right now on the show Game of Thrones they are supporting stories.