Just throw a USB stick on them and boot something like Linux Mint (defaults are pretty similar to Windows, should be obvious to anyone how to use it) to see how it runs with no installation and maybe showcase that to someone who could decide explaining briefly what it is and how it’s more lightweight.
I assume these have still some mechanical drives and that will probably be their biggest slow down. Upgrading to SSDs (which is still a lot cheaper than full computers) would bring them second life.
The i5’s have plenty of power for web browsing machines and can still be pretty snappy. Pentiums (4’s?) not so much, but it’s also worth trying