But I figured a lot of people did buy USB dongles for their desktop, i. e. "set up the old PC for the kids to do schoolwork in another room nowhere near the router." There should be bags full of them everywhere.

I actually have an 802.11b PCMCIA card sitting around. I bought it in 2003 because my university was rolling out a wireless network, accessible in about 1/6 of its surface area, and I was rocking a used Pentium II Thinkpad. Getting Slackware to handle 802.11 in those days was an amusing thing.