Iwasnt implying there was anything wrong with running aircraft on until the time out of hours or run out of cycles. As someone pointed out Lufty take the same approach with 747s as did AF with their 747 -100s .
These carriers would have done the math on getting the best economic solution and adopted that and I am sure that back in the day all three airlines provided as good a level of maintenance as existed anywhere to support the veterans. The T 7 seems a very well designed aircraft with an immensely tough airframe (BA at LHR and Asiana at SFO testify to that) so why not keep them in service, as long as the fuel economics are ok, for a good long period