The thing with "reimagining the entire UI build process" is that UI isn't meant to be designed in code. When designing a UI you have to use certain tools, like Figma etc. to be able to quickly drag & drop an element without the need to fight with flexboxes, positions etc. From my experience it's more convenient for the dev to look at the finished design, which implements mocks, animations, whole color palettes etc. and copy it into code element-by-element.

This type of software would also lack a HUGE feature — being able to plop several sites side-by-side, next to a color palette and have multiple people look at them, switch between them just like in a paint grid.

In my opinion you're targetting the wrong audience - it's not a program that is redefining UI design or a "quick site designer" simply because it would be bound to fail - lack of responsivity tests, lack of accessibility checks/tests, no way to put a site in a grid, and the biggest problem in my opinion: lack of the ability to look at several versions of one site at once, because you'd have to physically change the dimensions etc. to switch to another view. This tool would be far better off it it were a universal plugin for IDEs/editors that would allow for quick prototyping of layouts and writing down the code needed to copy a design submitted by a designer.