First 767 went into commercial service in 1982. First A320 went into service in 1988. If US ditches the 767s because of the age of the airframe design, it stands to reason the 320 series won't be far behind.
Assuming that Airbus cannot get their crap together on the 350 (likely), the best solution for US is to replace the existing 733/734 fleet with some combination of A320 series (sorry, BoeingBoy) and E190/195 (sorry ALPA), keep the 757s for long thin ETOPS missions and go with the 787 for everything else. Failing that, convert the 350 orders to 332 orders for longer/thinner oceanic ops and accept the inferior performance of the 330.
I agree..... with byfour!..... except I think 787 for LCC talk is just fantasy.