Speaking from experience working on an massive eCommerce site, minifying, concatenating and optimizing 2MB CSS file to 350kB-ish file resulted in an increase of mobile users. The improvement also added lazy loading for images and minifying JS and it boosted the number of mobile users significantly.
The problem was that users on mobile and slower connections would perceive the site as slow and unresponsive due to the massive file being loaded.
It's true that everything is well after that first load, but users might give up if they wait too long for that first load.
If you can play the Doom shareware episode in the size of your CSS you are doing something really wrong.
I've done quite a few audits of various sites in my career. I have seen some really crazy stuff I haven't thought of possible. Including the CSS and JS files with source maps included.
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