Use the system tools. But make the look of the system tools customizable.
System tool. Customizable look.
That's what should be available. But even today you can't consistently change something as simple as a scroll bar or a checkbox.
Which, even worse, are by default styled differently on different browsers.
Separating look&feel from functionality isn't that easy. In any kind of UX, they are bound to various degrees, depending on the use-case.
So if they're _exactly_ getting the point, they're pretty unobservant.
There already is no consistency.
https://en.wikipedia.org/wiki/Windows_Presentation_Foundatio...
Everyone, literally everyone is running their own thing. Name me big company that doesn't roll their own design system.
Preaching purism really doesn't help anyone, let's just accept reality as it is, and stop chasing some utopian dreams that just won't ever materialize (and if only because it would make a lot of jobs and professions useless, overnight, and there's too much inertia for that to happen).
We will never live in a world where an abstract concept like a checkbox can have an 'assigned' specific visual affordance that doesn't allow for adjustments. Such thinking is stuck in the past and won't allow for new/better UI paradigms.
To think that there's no room for variation in the design of form controls, given the broader context of a design system for app, is just a lack of imagination. I guarantee you that the people who design the operating system form controls (which change every couple years) have all sorts of ideas, and they end putting out one possible, relatively minimal option from many good designs they considered.