Internet Sharing turned on, but iPhones are "Unable to join"

This is a mid-2011 iMac running 10.13 High Sierra.

Its internet connection is shared via Wi-Fi, which worked fine for a few days, but then the iPhones in the house started saying "Unable to join".

Very similar to this question: internet sharing driving me nuts! - Apple Community

– except that the workaround of stopping Internet Sharing and changing the wi-fi channel didn't work for me.

One of the replies said that "internet sharing does not get much use," implying that it's ok for Apple to not maintain features that don't get much use.

I disagree. I'm in the software business, and we extensively regression-test software. If there is any regression in performance, the software doesn't go out the door. And we certainly don't have the resources Apple does, with its $2.6 trillion market cap.