This means that Zavala will likely die. We are supposed to get emotionally attached to him, but we already have Shaxx and Saladin, who could replace Zavala.

I'm not sure. I mean, it would seem likely to kill off an important character like how they killed off the queen (though we're not certain she is dead or not). But we can presume she is dead from the blast from The Dreadnaught. But the difference between her and I feel any other character they're gonna potentially kill off, is we never really got attached to her as a character. She was only present through a few missions. So if they kill off a main character like Zavala, it would be devastating I think. Also, in case anyone hasn't noticed yet, there's a part in the cinematic they showed of the legion we can presume blowing up the sun (a self made supernova) and it destroys from what we saw at least 2 planets. Though, we have no idea what galaxy that even is for now. But it's clear people will die in Destiny 2 because you can't take loss, without having death involved.

True, I think the explosion we see is just a retelling of the Cabal's power. I would respect and fear an enemy more if I SAW them blow up a star instead of my ghost just TELLING me they blow up planets for getting in their way.