The stronger stories don't go with that and that's part their quality. The unknown can be scary but when it's got no quantifiable factors it just ultimately becomes eye rolling. That's part of why Cold Forge, and Into Charybdis work, when you know in part what's going to happen when X approaches Y it creates dread.
So when you set up the rules in the first book then pay them off in the sequel, it is infinitely better storytelling than just doing whatever arbitrarily, and for one particular character that creates something that's genuine not just narratively but thematically.
The Pathogen does not affect flora in the films so to me that's a obvious contradiction, and it eliminates all the fauna, it does not create an ecosystem.