this is what I think. The change is that 10 of the highest-level weapon textures can be traded for a knife texture: the result is that the supply of knife textures goes up, but the supply of high-level weapon textures goes down significantly more.
It's not so much a depreciation of knife textures, as a distribution of this value down the chain of item rarities.
The broader impact is that it creates a lot of uncertainty around valuations in the market. This is probably the most impactful (on valuations) policy change made by Valve in the history of the market. Now there is an increased fear that more similar such changes may be coming down the pipeline.
It's not so much a depreciation of knife textures, as a distribution of this value down the chain of item rarities.