Interesting. I'm here after searching for "Restricted performance" as I have the same warning and reduced performance, but mine is intermittent so I'll open a separate thread.
Personally I wouldn't accept a failed turbo verdict unless they can provide any specific information. For me, main dealers are now nothing more than fitters, they look at a list of fault codes and are told to fit a new part according to what comes up. I had the same thing when I had a BMW, it was overheating and the main dealer told be it needed a new radiator. Having swapped that, with the fault still remaining it turned out to be nothing more than a stuck thermostat. I tried for weeks to get a refund but they insisted the stat had failed after they had changed the rad.
Regarding parts failure though, any part can fail at any time, it's an unfortunate fact of life but that's why we have warranties. The problem for the manufacturer/dealer is that there's no way to know how the car is generally treated for the most part, so you're never going to get them to give you a specific lifespan for any part. Of course you would tell them it's been treated with care, but then so would someone who had hammered it.