Note that there is a difference between retirement and destroyed. I believe my company still keeps an old coal boiler from the 1880s connected to a generator from the 1920s in operating condition. Because this generator exists and can be started in an emergency (with 12 hours warning for warm up time) we get a substantial discount on our power. I understand that about every 5 years the power company asks us to turn it on at which point we supply power for most of the town.

If you look you will see many businesses have their own generator running on the hottest day of the year - they get a discount. Because diesel generators can start on a minutes notice they get are higher on the priority list as to when they get asked to run on their own generator rather than grid power.

The above isn't to say that the power company is retiring these old power plants in the same way. I would guess in some cases the maintenance required to make them safe (or emissions legal) to use might be too high and they are destroyed.