In my opinion, 5-8 years degradation on grout at a spot that is used heavily, probably over 10 ten times a day, doesnt seem unusual. But I am not a tiler either...

One of the things I decided to do when we redid our kitchen is to be diligent about wiping down the area behind the sink. There is just no way to avoid water getting on that area.
So, I automatically wipe it each time I use the sink. My kitchen is going on four years and I don't have staining. Really, just takes two seconds out of my life.

I just talked to my GC and he commented that the degradation I saw was probably due to poor installation and cracking in the grout between the wall and the cabinetry/countertop. He said his method involves some sort of flexible connection between the two which cuts down on the cracking/water entering the cracks/rot. I hope he's right.

"Eroded and pitted" -- this sounds like the hallmark of incorrect water/grout ratio during installation. Adding too much water makes grout easy to apply, but weak. The grout in my bathroom is 83 years old, and is in fine shape.

The seam between counter and backsplash should be caulked, not grouted, and the grout can be sealed to protect from degradation by water exposure behind the sink. Angie has a great point about the grout mixture, too.