From what I heard from electrical engineers, the lifespan of electronics cuts in half for every 10C you subject to it. Of course this requires knowing what the lifespan of said part is for this figure to matter. For the sake of argument though, let's say I can get 5 years out of my CPU as-is, with a 50%/50% idle/load split to make things easy. And let's say the average idle temp is 35C. Assuming my math is right, having the CPU idle at 55C will drop the expected life span down to about 3.125 years.
Depending on how long your upgrade cycle is, running the fans to maintain lower temperatures is cheaper overall by virtue that if the fans break, it's a cheaper part to replace than if the CPU degrades out of spec and you have to replace it.
And if saving energy is your goal, turning off the fans is basically trimming the muscle. There are fattier things to cut since fans take like <3W apiece on full blast.
EDIT: Also if you get a quality fan, they last a very long time anyway. Noctua rates their fans for 150,000 hours, which is 17 years if you ran it 24/7.