A few points on that. First, in 25 years of front end dev at scale I can't think of a time when a site I've worked on has ever actually made a site wide change to the styling without it being a big deal. It's always a lot more work than just changing some CSS because you have to consider things like whether you actually want to change every button, what background color the buttons are sat on, what text color it has, whether the color you choose should be applied everywhere or just in dark|light mode, and so on. Site wide changes are a big deal and the actual code is a relative small part of that.

Secondly, with the componentization of web apps this is largely quite simple from a code perspective. All the other complexities remain but changing a Tailwind styled button isn't much hard than changing a CSS class.

Lastly, designers are moving to design tokens applied in Tailwind config or CSS vars. This decouples the colors from the classes and turns them into config that's exported from an app like Figma, with tools to automate that export as a git PR if you're feeling technical.

These things were hard to do. People in front end tech have thought about that and built tools to make it a lot easier.