I'm gonna make a second attempt to read that Bob and hopefully get schooled. On my first attempt I didn't get past the "...For example, a lot of portable radios are mono." In my old age have I missed some mono radio revival like the vinyl thing? Seriously....well mostly.

I do remember walking around with my little mono transistor radio in the late 50's.
Edit:
I read it all but still have difficulty with: "However, if you can get the mix to sound good in mono first, it will definitely sound great in stereo too." That seems to imply that you just hit a stereo button and presto the mono is stereo. But know you can't do a wholesale conversion in either direction. And then he said mixing in stereo is "easy." I guess that is very relative as that has not been my experience. I'm gonna dig deeper and see if some epiphany can emerge. I know I'm still missing something given the expertise of the proponents.
Edit:
Short but seems to hit on the primary reasons:
https://mixcoach.com/does-mixing-in-mono-really-help/He mentions how mono mixing can help understand (help eliminate?) sonic issues of having tracks stepping on one another. I can see how it would help there except for the always troublesome kick and bass that are, at least by me, mixed in the center in a stereo mix. Hey Dave, hope you don't think I hi-jacked your thread. Janice will tell you that I have a penchant for thinking out loud...often to a fault

Bud