Life
Classic's have anywhere from 10 to 20 years left on them, depending on airframe hours etc. Just have a look at how many B707's and B727's there are still flying around after years of service.
-400's... Those I bet will be around for much longer say 30 years.
Once they lived up their useful life in the first world there are eager 3rd world freight and even PAX operators happy to get their hands on these aircraft.
JJ