I think a definite figure is still unknown because of the use of new cone and surround materials, which were usually the limiting factor in age back when the main material for surrounds was foam, and paper in cones. Over time the capacitors in the crossover network may, or may not need to be replace depending on their age and type of capacitor that they are.
Of course, as with anything the way its treated and taken care of will also have an effect on the life expectancy.
I dont think 40-50 years is unreasonable for a modern speaker like the axioms.
