Finch could be replaced with Root. She has access to the machine and it would be a new interesting dynamic.
But the safer bet is on Fusco or Root.
Finch could not be replaced by Root. Nieto is right about no Finch equaling no show. He's wrong about no Reese, however. They both comprise the core unit and neither Shaw nor Root could stand in and see the show continue. It might make an interesting alternative for an episode or two, here and there, but right now, it would not be sustainable.
Root and Shaw are both still far too broken.
And currently, Root has almost no meaningful relationship with Reese and would serve as a poor replacement for Finch. Conversely, Shaw and Finch are still too uncomfortable with each other for her to replace Reese.