True, but that's probably about to change soon. Look at the ARM stuff about to enter the arena.
I wouldn't bet on us using just 2-4 cores 10 years from now. And we need to plan for that.
Because software doesn't really need many cores, no need to create CPUs with many, many cores.
I imagine the cycle's going to break at some point, after all we can only ignore having many cores for so long. Especially that now even mobile devices routinely have at least 4 cores.
As you note, the modal number of cores on mobile has been 4-6 for a while now.
The link you posted just indicates that Firefox rendered the page faster than Chrome; did it render it _wrong_?
One other note: the linked issue here is layout, not the CSS system per se; this part is not parallelized in Gecko and is still in C++, not in Rust.
Back when I was last measuring this, stylo's single-thread performance was comparable to Chrome's or a bit faster in some cases. Parallelized performance was much better.
That said, actual hardware in the wild has a surprisingly low level of hardware parallelism in practice. https://data.firefox.com/dashboard/hardware shows that for the Firefox user base as of end of Jan 2021 54% of users had 2 cores and 34% had 4 cores.