Yes, it would work, and yes, less stress would make the CPU last longer.
HOWEVER - at stock speeds, a CPU will always last a lot longer than a PC, even heavily overclocked, providing you dont fry it, it will still last longer than the life of the PC
a lot of newer cpus automatically decrease their clock speed when they're not being used (i.e. depending on the load)... e.g. if it has intel speedstep your os can do this, or sometimes even the hardware will automatically do it.