The A321 certainly displaced the 757 among the significant number of European holiday flight operators who used to operate it, who replaced their fleets of one with the other well before the US airlines have got round to the same thing over the same sorts of distances.
Regarding sticking to a single type, the traditional US majors have now merged up to such a size that the economies of sticking to a single type diminish, almost to the extent of going negative. If your fleet is two aircraft, it is grossly inefficient to have one of each. If it is 100 it is a lesser percentage difference to have 50 of each. If it is 500, which is the sort of size the US majors have now become, then the differential sort of disappears if you have 250 of each, plus the chosen manufacturer no longer has you by the short-and-curlies and knows it in negotiations. Notably, the larger Chinese operators, now with fleets getting up to US major size, have long done this dual sourcing.
Last edited by WHBM; 4th Nov 2013 at 07:38.