The thing is, this kind of issue is always going to be there, and is only going to become more significant as the entry price to produce 3P items and parts becomes easier. We are on the doorstep of home use 3D printers, and anyone with £1000 and basic web and design skills will be able to produce parts and add-ons and throw up an online store.
In fact, the wargame industry is actually well behind the curve with this kind of thing, and has been fairly insulated from it.
There is a huge industry of 'unofficial' Transformers products - both add-on and conversion kits, as well as full figures - which has grown hugely in the last 5 years. I own a few myself. There, they cater for the collector, a market that Hasbro largely ignores.
For example, this is NOT an official Hasbro or licensed product;

Games Workshop are going to have to take the same stance that most companies, such as Hasbro, do - They dont like the practice and they dont encourage it, but really there is not much that they can do about it while its a relatively small market sector. The best way to compete against this kind of thing - and something that Hasbro and Takara are starting to be aware to - is to produce these items themselves. In general, people will only go outside of GW if GW dont produce what they want, or charge too much for it. That doesnt make it 'right' or 'fair' or legal, but its the way it is. If both GW and CHS produce equivalent products at the same price, the majority of people would just buy the GW version.
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