London and New York are indisputable the places where English language publishing takes place. Traditionally, therefore, they were the places you needed to be to get the attention of a publisher. But while personal relationships still help, it would seem that there are far more avenues now than being invited to the right dinner party by the right hostess.
Which leaves us with the question of whether the company of other writers is needed to make you a better writer. Certainly, the company of other writers is pleasant to writers. I wish I lived in a place where it was possible to go have a drink with like-minded writers on a regular basis. (Like-minded is the hard part.)
But would it actually make me a better writer? Much as I have valued, or at least enjoyed, the experience of writing classes and critique groups in the past, and as much as if I were granted one trip in the Tardis I would choose to go to the Bird and Baby for an Inklings meeting, I am less and less convinced that any of this actually makes me any better. I think you learn to write by reading with attention, but that fundamentally you learn to become a writer by learning to see -- by paying the proper kind of attention to the world around you, to its texture, to the particular difficulties of human life and human relations.
And those things you can do anywhere.
Or, to put it another way, writing is participation in a conversation extended through time and through space. It gets is particular character not from presence but from distance. It is not only a lonely profession, loneliness, and the yearning of the lonely spirit across the gaps of time and distance, may be of its essence.
Or that maybe a piece of romantic puffery and fiction may not be any different really from marketing writing, in which case, iron sharpens iron, and if you want to excel, get you to the place where iron meets iron, which is London and New York.