15" is deep enough

I would be a little more lenient, say 18 inches. Though I get the point you are making,.
As far as being the benevolent despot goes, I would make it illegal for cable manufacturers to make a speaker or interconnect cable that cannot bend less than a 3 inch radius. (I'm looking at you Transparent!).

I've been there. So, I built my "final" rack, and made sure that the bottom shelf was 22" deep to handle those mono blocks that I would someday possess. Well, my "final" mono blocks turned out to be 15" deep. - LOL
Kenny

Good luck with that one.

All is good!