> If one already has an (non-React) SPA which is not already built following Redux then you're stuck with what you've got;

This is simply false and is outright wrong. This even goes against the most basic React+Redux guidelines. Dan Abramov, one of the creators of Redux, is quoted in Redux's FAQ:

> would like to amend this: don't use Redux until you have problems with vanilla React.

https://redux.js.org/faq/general

> it isn't really possible to take an existing SPA of any complexity and deciding to "make it use Redux"

Frankly, you sound as if you have absolutely zero experience with React. I know for a fact that a few flagship SPAs from a few companies, including some FANG webapps, do not use Redux.