Old airliners are like old washing machines and cars and the like. Why would you wan't to still fly a tired, old inefficent plane like a (what ever!@##) that has been maxed out as far as hush kits, new interiors, flight deck etc when you can lease a new(er one for less money with lower operating costs? I think the passengers would rather fly in a state of the art A320 or 757 offering the latest in accomidation, comfort and safety as opposed to an old 707 let's say. And the Airlines would rather fly new as well. A pair of CF6's, RB-211's etc are a whole lot cheaper to run and maintain than Conways or JT3D's. Don't forget old planes have more down time than newer ones too. Besides there are explicit limits to how long a plane can fly.
Planes in most ways are a self renewing resource. Old ones are stripped to make spares for other's like it (As oppsed to making new replancement parts for old planes) and once the airframe has been stripped, the metal is recycled for alot of things, not just cans of Bud Lite. Not only that, before break up, lots of old PAX jets server on for a number of years as freighters, ekking out the most from the plane.