Some maintenance is deferrable, but the Minimum Equipment Lists and other rules govern how much any airline can legitimately scrimp. iirc, when PA sold its Pacific routing to UA, the aircraft included had lots of deferred maintenance items to be taken care of at the SFO base - on one a/c over 900 items, again iirc. But when it comes time for some of the more critical operations, checks or parts replacement, the clock can not be stopped or ignored; that's one reason some 762s are sitting at Roswell, NM.

(See the new thread for more about the 762s at Roswell being sent to scrap.)