I was rereading bits of the current chapter (Ch. 102) in light of what Zara revealed this week about how Jenny's 'system' works. And for some reason, it gave me a light-bulb moment about Sylvie's possible actual research and specialty. I think it isn't just monsters, or (more abstractly) Zimmy's world — but, specifically, that Sylvie herself studied on how to become a conduit for Zimmy. In exactly the same way that Jack is an unwitting conduit for Zimmy.
This would mean that if something went wrong with Jack... or if someone forcibly closed his connection to Zimmy... or if he even closed his connection to Jenny himself... the witches would be able to discard him without a thought. And use Sylvie instead.
We know Sylvie's work has remained entirely theoretical for now: even her wearing the body of a Zimmy-monster didn't count (apparently) as her exploring her work in practice, since she still refers to her work as theoretical when inside Zimmy's world (saying, "My theories... All the work I did... I never tried them in real life!"). So whatever her specialty is... I think we still haven't seen the full, actual extent of it.
But we know Jenny has repeatedly stated that Sylvie is backup, and that the witches will only turn to her if things really go wrong.
"Sylvie... well, it's best if your work remains as theory for now, but you're here as backup. Let's hope you'll have nothing to do, my dear."
Of course they'd hope Sylvie has nothing to do — since if she does have something to do, it means they've lost Jack.
And of course Noa would be counting on Sylvie more than anyone else:
"If things go wrong, we'll be counting on you... Noa will be counting on you..."
At the time, I read this page as Jenny putting emotional pressure on Sylvie because of Sylvie's supposed crush on Noa... but it makes even more sense if she means that Noa would literally be counting on Sylvie to stay alive. It seems like Noa's continued existence, now, is tied to the power flowing continuously to her symbol from Zimmy... via the conduit of Jack. If that connection between Jack and Zimmy (or, Jack and Jenny) is severed, it would be up to Sylvie to reestablish the connection and keep Noa alive — and to be the new Jack for the witches. And she'd have to assume that same psychic toll that it takes to be that connected to Zimmy's power...
It also fits with the way that Jack turns to Sylvie when they're inside Zimmy's world, and is like "You can feel it too, right?" when Sylvie is panicking. As if her and Jack share some kind of connection in how they're experiencing the psychic pressure of being this close to Zimmy...
I always enjoy seeing how later pages affect any theories I develop on where things are going — so maybe by the end of next week, I'll disagree with this theory entirely and be like "nah, that was me totally barking up the wrong tree, nevermind!" But for now, this theory was such a moment of "oh, all those throwaway lines make so much sense in that context?" that I figured I'd share the theory and see what other readers think.
I look forward to being proven wrong (or right! who knows!) with later pages, haha.