Itche, Most Rishonim indeed lived in the Ashkenaz area, but that is no proof that Bavel moved to the Ashkenaz area or that Eretz Yisroel moved to the Sefard area.
There is no concrete evidence anywhere that exists indicating where the Yidden from Ashkenaz mostly came from (prior to Ashkenaz), or where the Yidden from Sefard mostly came from (prior to Sefard). If you believe otherwise, please document where such concrete (rather than speculative) evidence exists. It does not.