> Redux only plays a role in adding syntactic sugar over React's support for reactive programming

Redux doesn't have anything to do with any kind of syntactic-sugar for Reac.

> the general guidelines are that you should only bother with Redux if your SPA is already considerably complex

If one already has an (non-React) SPA which is not already built following Redux then you're stuck with what you've got; it isn't really possible to take an existing SPA of any complexity and deciding to "make it use Redux" because it completely up-ends practically every aspect of SPA application-design, at least as far as how "traditional" (non-React) SPAs do things.

If you'll permit me to oversimplify: I tell people that React is a basically an opinionated Redux kernel wrapped around a JSX-based DOM+component/widget system (, which then means you now need a JS build system too); or conversely: Redux is like React, except without JSX components.

...or am I misunderstanding you?