Byrned: I find it amusing that you're willing to use the argument that "the manufacturer says it should last this long so the CGA applies". And then you quite willingly say that you don't care what the manufacturer says, it should last this long. I guess we could sum it up by saying xxx item will be covered under the CGA for however long the manufacturer states or what is a reasonable time period, whichever is the greater.
I'm not saying that it doesn't matter what the manufacturer says. What i'm saying is that what the manufacturer says is often way understated, and one of the main reasons the CGA exists.
The 'reasonable time period' you mention takes into account what the manufacturer says already. The law exists to protect the consumer from manufactures understating lifespan, not to protect the manufacturer or retailer if they overstate the lifespan. Otherwise we would have everyone selling everything with a 'lifetime warranty' but then refusing to honour it. The law says the consumer can pursue the retailer OR the manufacturer to enforce their rights in this case.
If we put aside the 20 year warranty for a moment, and look at what we might consider a reasonable life span for an expensive knife, we may come to the conclusion that it was less than 20 years. That is a perfectly reasonable conclusion to come to, HOWEVER, when we then take the 20 year warranty into account it then becomes an additional selling point for the product in the form of an assurance of superior quality. Both the shop and the manufacturer benefited from that (in the form of sales), and the law says they must stand by that claim.