In this context, the question is not about “useful life.” This is the length of time an airplane can economically be in service before passenger carriers retire the aircraft. Then there is the potential as a cargo conversion airplane. The useful life may equal or exceed the useful life as a passenger airplane.
How much life is left in the MAX in this context means how long will it be before Boeing pursues a replacement design—and how long will MAX remain in production?
737NG program launched in November 1993. EIS: December 1997. Production ended late 2019.
737 MAX program launch, July 2011. EIS: May 2017. Boeing contract with Spirit Aerosystems for fuselages extends to 2033.