So what makes you think that an all-new aircraft 30 years further down the road than the A320 wouldn't be able to offer a corresponding leap over it in technology and capability ?
A brand new design today could offer some small improvements over the A320 but it's doubtful that there would be the massive leap ahead which would be needed to justify the massive cost of a brand new aircraft and the necessary price increment over the A320. The Airbus can use the latest engines and with sharklets has a very efficient wing, the fuselage is the right size and with additional centre tanks range can be extended.
A few tweaks here and there and the A320 can easily go another 20 years, by then technology will have improved to the extent that it will be worth incorporating the advances in an all new design which will probably be all composite and use half the fuel of the present aircraft. The B737 should have been scrapped in the 1980s and replaced with a design using B767 level technology. Had this been done Boeing would have had an equal competitor to the A320 in the narrow body market .