I accept that the very few who make significant edits to our core CSS will find this change uncomfortable. The vast majority want to add or override existing CSS, so this change will not affect them.
We want to get to a point where we can serve CSS from a single set of files via a CDN. We also want to overhaul and modernise our CSS for efficiency and enabling a more streamlined output from a better build process.
These things do take time, and we have to take steps towards that goal, and drawing a line with 4.7.0 and asking that core CSS is not edited is the first step towards this.
As a temporary measure, a template edit in core/front/global/includeCSS to remove the framework CSS and instead link to a separate directory of your own optimised CSS would enable you to carry on with your heavily optimised CSS. I can help you with this if you wanted to explore this.