How many 747s have achieved 30000 cycles? Seems quite a reasonable limit (or target!) but with an average sector length of 4? hours only the very high timers will be affected anyway. The obvious exception to this is the Japanese SR varients of course.
I thought that it was economics that always grounded aircraft and not engineering. Modern aircraft can fly on almost indefinately now if maintained and modified correctly, but there comes a point where the financial cost for this to happen exceeds revenue and they become unprofitable. Interesting rumour nonetheless, if older a/c are to be phased out at a definite point (and maybe this is the thin end of the wedge) then is Boeing just trying to create a market for its new aircraft??