Frameworks like Knockout have been around for quite some time now. You don't have to use React to not depend on the DOM. There are many alternatives to "jQuery based front-end development" that's not React. Aurelia, for instance, happens to be an amazing framework in my opinion that's also highly testable and that's not React. Like I said, modular code is modular code. You can write good, modular code with just require js modules, and you can also write terrible monolithic React components.

Testability isn't the domain of the view layer.

Abstracting the DOM into a declarative DOM is great for performance, but doesn't lead to necessarily more testable code.