The failure mode is not as other posters have said mechanical, but is in fact usually a business decision. Plenty are lasting much longer than 25 years.

Many wind turbines were built with subsidies to run them for 25 years. As soon as that time is up, they may no longer be worth running, especially since in many places wholesale power costs have dropped due to other subsidised wind and solar.

Also, finding sites to build wind turbines is getting tricky - you need good wind, no complaining neighbours, good road and crane access, and a good grid connection. That usually makes existing wind turbine sites excellent candidates for building bigger better turbines which are 10x the size.