That your P90 is slow to run modern software is largely because modern software is bloated. You mention:
> mpv and mplayer can't play anything smoothly (not even as a .wav file) and mpg321 fires up pulseaudio before it starts playing so that's not happening.
But a P90 could not only play wav files (in fact my 386 with 4MB of RAM could play and edit wav files in realtime just fine under Windows 3.1) but also both decode and play mp3 files at the background while doing other tasks in Windows 95.
That mpv and mplayer are not able to do that shows that they require way more resources than is really necessary to perform the task at hand.
> People aren't being foolish or irresponsible or naive in not caring about these things, they're being sensible. Running a Pentium 90 with modern software --- that's the actual unreasonable hobby here.
That is a false dichotomy, running modern software on a P90 might not be very reasonable but that doesn't make people who do not care about the resource usage of their applications sensible - if they are or not depends on other matters.
And being unable or hard to run modern software on a P90 is because of the state of current software, but that doesn't mean software has to be like that because it is "modern" - someone explicitly made a choice to develop the software in a way that causes it to be like that.