It's definitionally not premature optimization. Pipes exist (and have existed for decades). This is just "optimization". "Premature" means it's too soon to optimize. When is it no longer too soon? In another few decades? When Linux takes another half of Windows usage? It would be premature if they were working on optimizations before there were any users or a working implementation. But it's not: they're a fundamental primitive of the OS used by tens of millions of applications.
The tradeoffs you're discussing are considerations. Is it worth making a ubiquitous thing faster at the expense of some complexity? At some point that answer is "yes", but that is absolutely not "When it's easy and has a huge benefit". The most important optimizations you personally benefit from were not easy OR had a huge benefit. They were hard won and generally small, but they compound on other optimizations.
I'll also note that the Knuth quote you reference says exactly this:
> Yet we should not pass up our opportunities in that critical 3%
The tradeoffs you're discussing are considerations. Is it worth making a ubiquitous thing faster at the expense of some complexity? At some point that answer is "yes", but that is absolutely not "When it's easy and has a huge benefit". The most important optimizations you personally benefit from were not easy OR had a huge benefit. They were hard won and generally small, but they compound on other optimizations.
I'll also note that the Knuth quote you reference says exactly this:
> Yet we should not pass up our opportunities in that critical 3%