WebKit is also used by GNOME Web/Epiphany and more embedded applications than can possibly be counted. It fits a lot of places that Blink doesn’t because it’s built to be highly embeddable (more than Blink is anyway) and has bindings for every language imaginable.
I noticed that none of the cross-platform frameworks listed here https://xpda.net/ use WebKit.
I am guessing many commercial users of WebKit can be guessed by looking at the list of Committers (of which Igalia has quite a few): https://webkit.org/team/
