Are RPGs expected to contain lots of art? I'd like to examine that starting point. I admit I only skimmed the article but I didn't see any analysis of if less art means less sales. I wonder if people are trying to adhere to the standard set in games like D&D and PF, but those have art because they can, not necessarily because they must.
Blades in the Dark is extremely highly regarded and has some art, but certainly not tons of it. Microscope and Kingdom haven't got any, as far as I can recall.
Blades is an interesting case. At least when I ran it, I mostly used the quick start guide/starter kit, and only used the book as reference. As for reading it cover to cover, I think art does admittedly break up sections better than type formatting ever can.
Using art to break sections up is probably its only practical purpose, and I agree it does help with that, but IDK if it's as core as the author of the article seems to take for granted.
Sure, but it's easy to make a rule of thumb, harder to interrogate it. Why should there be that much art? Again, microscope and bitd definitely don't follow that rule, and seem to be successful.