[formerly_proven]

I included the structure because I only know of cases (including my own) where both needed replacing. Centuries for the wood structure is hypothetically possible of course, but only if maintenance is ideal (no water ingress) and without insects. Once there are water issues, they rapidly decay. Retrofitting both new insulation and additional roof loads like solar is also not possible for every old roof structure. The flat roofs somewhat common in 60s and 70s construction also have very short lives and often need to be replaced wholesale after 40-50 years. Despite the reputation, they're not at all cheaper than traditional roofs, at least not when you have to adhere to the current building codes