Mercury in the bulb will be destroyed, absorbed eventually, the lifespan is mostly a how much they put in the bulb.
Also the phosphor also degrades and is destroyed.
120v->ballast->Mercury vapor->UV light->phosphor->visible light
In every CFL there are 3 things that WILL go eventually. The ballast (depending on quality), The Mercury (depending on quantity) and the Phosphor (also depending on quantity).
So it's quite possible to make relatively long lasting CFL bulbs, but in general they will just try and meet the requirements they market them at.