I understand the frustration at limited life cycles for expensive airframes, but let's try to be real about this: there will always be some limit - the only issue is - what number?
Your basic 747 is a few million pieces of something or other, carefully stiched together, all wearing out at different rates. Some pieces are near the surface and designed to be heavily maintained, others are not so easy to reach. Yet others, like the slowly crystalizing alloy of the bulkheads, pylons, fuse and wing beams, etc., can wear in ways that are very hard to assess; harder yet to predict. If you think the book service life numbers are perfectly true for all cases, then you oughta be walking for a living.
At some point in an aircraft life cycle it begins to take greater skill, money, luck and expertise to support an hour of seat miles production from a dear old aircraft than from a shiny new one. The 'loving care' quotient - and operating expense - goes up from then on until the boneyard takes it.
The spec goal that 100 percent of passengers should reach their destination alive for 100 percent of flights imposes some difficult maintenance constraints. Surely there is some tradeoff. Perhaps if that unreasonably stiff criterion were reduced to 99 x 99 percent, then some aircraft could go out to 500,000 cycles. But would net profits increase as a result? Probably not, because the hard-core fatalists represent only a small fraction of the flying public - at least among the people in the paying seats. The folks who mostly hanker to sip a mai tai while burning on a beach somewhere do not like to think of that as a calculably risky activity.
An airframe manufacturer's engineering people are in the uncomfortable position of knowing that certain things in the way of design now can be - and should be - done differently and 'better' than they were 35 years ago. It is hard for them - mostly young f**ts anyway - to justify extensions of service life for machines they perceive as obsolete and incurably fallible in some respects.
IMHO: Yes it is a conspiracy by Boeing to obsolete old aircraft - no it is not a totally bad idea.
Besides, the time-building pilots need something to fly in the Andes at night hauling produce and hot suv's for pennies per hour. Turn the old birds into freighters on the way to retirement, and put them out where they cannot do very much harm.