I don't understand your motto "Make Bitcoin Decentralized Again" ... When it became centralized? 
What does it means?)
What does it means?) It means giving the power back to
FTFY
AMD & nVidia aren't miners plus there are many more GPU manufacturers than ASIC manufacturers AMD & nVidia may produce the actual gpu core but it's the many manufacturers that release the cards.
You are not forced to get your cards from AMD & nVidia there is nothing stopping you from going to your local big box computer store
...and buy card containing chips made by AMD and Nvidia.
Nevertheless: I find the entire 'bubut muh centralization' narrative ludicrous. As long as there are no prohibitive institutional barriers to mining entry, it is as decentralized as is beneficial.
The issue is not the chip manufacturer making the chips its with the chip manufacturer trying to dictate the network
there is no way either AMD or nVidia can outrank everyone that owns a GPU they struggle now to keep up with the demand of the gaming market because of miners buying cards
Plus the cards can remain profitable for a very long time I have a R9 280x here that although is now must be going on 5 years is still working fine and profitable it's not as efficient but had paid for itself many times over but the majority of what I have at RX480s
