hmmmmm, then the below is correct usage? I am certainly no expert on grammar, lol. And I avoid contractions as much as I can since I was taught (again back in grade school) that contractions were for lazy writers. I expect things to heat up if I understood you correctly and the below is correct usage. I look forward to getting a lesson on English grammar.
the world’s richest city
Yes— world’s is correct usage—worlds’ would indicate that the city belonged to multiple worlds.
My grammar is by no means perfect—I’m only 43 and things had slipped a great deal by the 70’s and 80’s [another usage of apostrophes], but as part of my job involves grading papers I keep a few books handy to refer to when I am inclined to use the red pen but have doubts.