These planes will still be flying in the 2030s.
Since these planes won't suffer from metal fatigue like planes made out of aluminum, that means that they'll last longer?
Metal aircraft don't necessarily have to suffer so badly from metal fatigue that they have to be replaced inside of 15-20 years. Fatigue depends on usage patterns and there are 747 still flying after 30 years of regular use and with good maintenance should be able to last at least the better part of another decade. USAF engineering studies project that their B-52 fleet would not reach the fatigue limits of it's wing structure until the 2040s but keep in mind these B-52s do not get flow as hard as the 747. The B-52s that are now in service left the factory in the mid 1960s. An American airforce veteran I met a few years ago told me that there are actually cases of the third generation of soldiers from a military family flying B-52s. Dunno if that's true but theoretically it sure could be. Just about the only criticism you can throw at the B-52 is that it could do with an upgrade to more modern fuel efficient engines which Boeing estimated would increase it's already impressive loiter capability by 46%.