I would imagine that from an architectural point of view most towers are, like the majority of other buildings, designed to stand indefinitely. Generally new towers are constructed not because of any structural issue with the previous buildings but because the progression of technology and expansion of airport operations has rendered it necessary. Heathrow is a good example, I don't think there are any structural issues with the old tower (the building is still in use) but the expansion of the airport's operations led to the necessity of the development of the new tower.