I also think you might be ignoring some important factors — mainly the problem of CSS maintenance.
I think a lot of people could relate to this quote from a fairly recent css-tricks article about the whole modular css movement
CSS grows over time. Smart people on great teams cede to the fact that they are afraid of their own CSS. You can’t just delete things as it’s so hard to know if it’s absolutely safe to do that. So, they don’t, they only add. I’ve seen a graph charting the size of production CSS over five years show that size grow steadily, despite the company’s focus on performance.