It's probably not common, but for me the answer lies here:

> I don't recall seeing them used as substitutes for classes or vice versa for years at this point

I had a fairly large school project developing a JavaScript application back before ES6, and kept searching for a better way to structure my code because I found it awkward using prototypes + binding this for events. Eventually I found a hidden golden nugget of a tutorial that showed me how to do it with closures and I was in awe that it somehow wasn't the norm (and obviously how cool it was to build objects out of functions!)