1. Why hire a team of non-webdevs to do webdev. (on 2nd read, it appears they wanted to use the same team from the WPF app... ¯\_(ツ)_/¯)
2. If you build and treat React components and their APIs as the abstraction layer, then they're just components... who cares how they're implemented.
3. Just because the existing project is big, doesn't mean your new one has to be... split it up!
4. Redux... the root "technical problem"
My hot take is that the authors technical issues probably come from Redux and not React. Using redux as the core of your application is like pouring glue on a lego set. In an ideal world this is great because everything is strong and well defined. In reality, it prevents flexibility and applies hard constraints to the entire system.
That one choice of using redux effects the decisions you make in every component. You lose flexibility to encapsulate features and experiments. You're forced to bend over backwards to do things in specific ways.
Redux is the JS equivalent of the Windows Registry.