This position doesn't make sense to me. Certain customizations to the native control (which is the ask here) are already available, such as adding padding to input controls, increasing the font size, changing the width. You can even change the border color and the border radius and the background color for some controls! Some of these turn out to be necessary functional changes, and some of them -- imma say it -- are obvious and important adjustments so that your app looks consistent and everything lines up properly.

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.

Please have a look at any OOP desktop framework from like the past 3 decades. It is not a difficult concept — one is free to overload the render function, while the functionality will remain the very same, like getting focus with tab, activate to space, whatever.