Worth noting that 100 years is not exactly the hard deadline, but a typical (default) *target* for the calculations made by the engineers planning the construct

Worth noting that 100 years is not exactly the hard deadline, but a typical (default) *target* for the calculations made by the engineers planning the construction. What I mean is that this number is defined by design, although they are of course also constrained by what is possible within the set budgets. Reinforced concrete lifespan depends on a lot of factors, including some that can be controlled, such as isolation of the reinforcement, materials used, various qualities of the concrete itself. When the new structures are designed, engineers do the calculations to achieve a specific goal in terms of lifespan, construction cost and maintenance cost. I don't know how this works in practice, but I would hope that various factors such as the total cost and environmental impact (yes, I can hear my inner self laughing at my naivety here) are taken into account to arrive to the optimal target.