React was advertised from the start as the V in MV-whatever, so that seems like an overly restrictive definition of "idiomatic".
> At which point, JSX is basically just another fancy HTML templating library.
JSX is optional syntax sugar; my team uses React and Redux heavily, but we've opted not to use it.
HTML templating is great actually, all the rendering is data -> UI.
If you go back to the early Rethinking Best Practices presentation[0], it boils down to asking why client side development isn't like like HTML templating. The answer is that updating the entire page DOM on every change is unworkable, there needs to be support for partial updates. That's what React offers, a return to the data -> UI paradigm, without having to write a secondary code path for partial updates.