I'm pretty much convinced this is what they did. Either that or:
- The final cutscenes show the "Zack didn't die in the struggle" timeline and the plate-fall aftermath of that scenario.
- Biggs & maybe Jessie were retroactively revived in the same timeline as us, because removing the ghosts across 'all points in time' retroactively nullified certain developments we saw happening in the plot. Go back the furthest you can to ask yourself "so what if the Ghosts did not stop this" and think about just how many parts of the story could change, not completely, but slightly, but without affecting Cloud/Co. because they were stuck in the singularity as the ghosts were being killed in 'reality'.
I also think that they're doing a little trick with Jessie. They show Biggs surviving, but they show Jessie's wear on the table. Her plated gloves and headband which is to play with the idea that "maybe she also died or survived?" but what I think it really leads up to is that,
if Jessie didn't die in this altered timeline, she will have left her belongings and moved out in the world, because she has decided to take her acting career on Golden Saucer seriously, and we will then encounter here there, in Part 2.
Last trick I think they might be doing is, if you consider what happened when we entered the singularity, remember how at first Cloud and Co. went through only to arrive at the same road in Midgar? I think they just stepped into the "Remake" timeline, and that's what we will follow from here on out. Then, the visage of Zack surviving and other things, maybe also BIggs surviving is everything that we left in another timeline. Then at the end of the final part of remake, the reward for saving the world in the new timeline is that we can go back to the timeline when everything was basically saved earlier on. Idk, just ideas.