it's not impossible for large and complicated apps to end up with over 4K selectors
Is that 4K of selectors that are all actually used, or are most of them just taking up space? If it's the former, then I can see the concern, but if it's the latter, then maybe the 4K limit is perfectly reasonable after all, and instead we should be rethinking how we generate our CSS (a quick search shows plenty of tools that will find and remove unused selectors.)
Or put another way, it's the difference between complaining about some system not having enough RAM instead of trying to rewrite your application to be more efficient so it can stay within the limits; and I think most recently there's been far too much of the former and not enough of the latter.
Sure, you could remove unused selectors, but then you'll end up with different CSS files for different pages. That's a lot of extra HTTP requests for a first-time visitor who wants to look around your website.
Whether or not the extra HTTP requests are worth the time and RAM saved by removing unused selectors will probably depend on your specific needs and priorities. For the time being, though, I suspect that modern computers take less time parsing a few thousand unused CSS selectors than the time it takes to fetch an uncached HTTP resource from a few thousand miles away. Moreover, devices with less RAM and computing power tend to be connected to even slower networks (e.g. 3-year-old phone on a 3G network).
Is that 4K of selectors that are all actually used, or are most of them just taking up space? If it's the former, then I can see the concern, but if it's the latter, then maybe the 4K limit is perfectly reasonable after all, and instead we should be rethinking how we generate our CSS (a quick search shows plenty of tools that will find and remove unused selectors.)
Or put another way, it's the difference between complaining about some system not having enough RAM instead of trying to rewrite your application to be more efficient so it can stay within the limits; and I think most recently there's been far too much of the former and not enough of the latter.