Technical achievement aside, when a person invents something new, the burden is on them to prove that the new thing is a suitable replacement of / improvement over the existing stuff. "I'm starting to view /not/ using [cosmo] in production as an abandonment of professional responsibility" is emotional manipulation -- it's guilt-tripping. Professional responsibility is the exact opposite of what she suggests: it's not jumping on the newest bandwagon. "a little rough around the edges" is precisely what production environments don't want; predictability/stability is frequently more important than peak performance / microbenchmarks.
Furthermore,
> The C library is so deeply embedded in the software supply chain, and so depended upon, that you really don't want it to be a planet killer.
This is just underhanded. She implicitly called glibc and musl "planet killers".
First, technically speaking, it's just not true; and even if the implied statement were remotely true (i.e., if those mutex implementations were in fact responsible for a significant amount of cycles in actual workloads), the emotional load / snide remark ("planet killer") is unjustified.
Second, she must know very well that whenever efficiency of computation is improved, we don't use that for running the same workloads as before at lower cost / smaller environmental footprint. Instead, we keep all CPUs pegged all the time, and efficiency improvements only ever translate to larger profit. A faster mutex too translates to more $$$ pocketed, and not to less energy consumed.
I find her tone of voice repulsive.