Where did you hear that reinforced concrete has a lifespan of 100 years?
As a structural engineer, I can tell you this is not correct as a general rule, but is true for structures subject to chlorides, especially marine structures like bridge piers.
One of my now-retired professors at the University of South Florida studied concrete durability for FDOT. He told me FDOT is now using a 100-year design basis for bridges. The concrete materials and additives have gotten quite good over the past 2 decades. We are learning a lot and still improving our concrete.
The key is to make a tortuous path for the chloride ions so it takes them decades to build up enough to overcome the passive film at the steel-concrete interface. The high pH of the concrete matrix causes this passive film, and it takes either acidification or chlorides to defeat it. Concrete bridge decks and roads in cold regions that use deicing salts are also damaged by chlorides.
Reinforced concrete protected from the weather in buildings would not have a 100-year lifespan forecast. If the building is properly maintained, the concrete should last much, much longer than that. I say "properly maintained", because of the Surfside Condo collapse in Florida. There a leaking plaza deck, lack of maintenance, and design flaws (columns too skinny) led to a tragic collapse of a building.