сильные мира сего = the powerful (people) of this world
An article on Wiki says that the demonstrative "сей" sounds outdate, literary, humourous etc and is rarely used except in certain set phrases.
In this specific expression, how does the genitive "сего" sound, as opposed to saying "сильные мира этого" or "сильные этого мира"?
"Сильные мира сего" is a set phrase, so "сего" sounds perfectly normal, while "этого" sounds unusual and weird.
It's a stable collocation, originating from an earlier period, therefore it sounds normal within this specific phrase. And as a Russian proverb has it "there's no taking words out of a song"
The phrase appears to be of a Bibleic origin and one author claims originally was "великиемира сего".
It's a bit like trying to change "the powers that be" to "the powers that exist". Сильные мира сего is a phrase that carries a specific feeling with it and is used in certain contexts just like powers that be.
