I'm not aware of any expected building lifespan. That they started with 50 years for
DVC and kept matching the 2042 expiration date wasn't because of an expected lifespan on the buildings. It was likely a combo of things, most importantly that they were able to take back control of prime real estate at a point that the time was appealing enough to sell but of course not too long that they were locked in for a century like Swan/Dolphin.
CR and Poly are approaching 50 years old and it descends from there and no plans for demolition. They sold 30+ year old buildings at the Poly for a 50 year time. They certainly might choose to raze and rebuild DVC resorts as they expire but it's still a lot of years between then and now and will depend on what the Disney Company is up to at that time.