DaasYochid: Why would you assume the differences are lessened in each generation to the point that future generations would be able to intermarry with ease? The religious minhagim will still play a large role. Would an Ashkenazic wife want to assume very different minhagim? (Or vice versa.)
Chasidim and Litvaks have been living together for hundreds of years including in America and Israel mixed together for many many decades. Do you hear of Belzer Chasidim marrying Masores Bais Yaakov girls or Lakewood bochorim marrying Vizhnintzer girls?
Of course there are some such (and some Ashkenaz/Sephardic) marriages. But it has never become the norm.