If they never need sharpening, then you’d never need a new one resulting in a tremendously negative impact on manufacturing.
Tools which cut materials are the heart of manufacturing, mining, farming, all sorts of things throughout the productive economy. A great deal of time and money goes into keeping those tools sharp. And a whole lot of productivity is lost by dull tools.
I don't know about you but I don't buy a new knife simply because it needs to be sharpened. Self sharpening knifes would wear the blade down just like a manually sharpened blade does, the difference being the owner would never have to do it. Eventually they will wear out.
Not necessarily. Serviceable cutlery can last for decades - my parents' has - but that durability hasn't stopped it from staying cheap and plentiful.
One of the main sources of demand is new households. Because cutlery is cheap, using hand-me-downs seems miserly. Typically, buying a new set for a new household is preferable. Even in these days, there are many youngsters still moving out of the nest.