10 Reasons to believe that Phasma is still alive
1 - Her armor is made of the metal of General Brendol Hux’s spaceship. It resists EVERYTHING, including fire.
2 - As the whole ship was being destroyed at that moment, there could be something bellow the fire for her to fall upon. You can’t say she fell in nowhere because we don’t know if there was something there or not.
3 - She has experience on escaping situations where the odds are against her. Let’s remember how she escaped Starkiller base while it was exploding in her comics.
4 - Who’s left on the First Order if she dies? Kylo Ren and Hux can’t rule it together, since they pretty much want to kill each other. If Hux is going to take Kylo down after discovering he killed Snoke (which most fans believe will happen), who will help him if not Phasma? She’s helped him killing his father already. And if Kylo is going to take Hux out of the way, who will help him rule the First Order after that?
5 - She can both stay in the First Order of betray it, the two things would create an interesting plot for Episode IX. In her novel, chapter 13, page 95, there’s a line that says “she’ll one day betray the First Order like she betrayed everything else she’s ever professed to love”. Considering that Star Wars books generally give clues on what will happen in the movies, this line may be prophetic.
6 - She’s the most important woman in the First Order, and there are not many more as far as we’ve seen. Also, she’s the first female villain we see in Star Wars movies. Killing precisely her would be stupid, don’t you think?
7 - There are infinite references they can make to her novel in the movies, related to other characters we don’t know much about, like Armitage Hux himself, and the beginning of the First Order, which we still know only briefly. That would be something fans would like to see in Episode IX.
8 - Why would they choose Gwendoline Christie to play Phasma if she wasn’t planned to have a minimally important role in at least one of the three movies? If she would just be there saying only short lines behind an armor ans die soon after, they could have chosen anyone, and it would be probably cheaper. If they chose her, it can only mean one thing: she’s going to be important, even if only in Episode IX. They always keep the best for last, right?
9 - If they were really going to kill her, they’d have kept the alternate/deleted scene, because there we can see more of her and of Finn. Yet, they changed it for something a way quicker, in which we barely see her talking, much less the fear in her eye, other troopers discovering what she did on Starkiller base... This probably means we are going to see all of that later on.
10 - Many fans were annoyed by her short appearances and lack of role, mainly after reading her novel. Possibly killing her made them even more angry. Keeping her dead for real would do worse. Now bringing her back and showing us the amazing Phasma we read in the novel would probably fix all of that at once.