Every product Bitmain ships is torn apart and analyzed by many people world wide. If such a deal was occurring we would know.  I didn’t say it was impossible, just that we don’t see it happening and there are strong market reasons why it wouldn’t.

The best way to mediate the risk of centralization is a properly functioning free market. That may be an illusion, but the entry of more hardware capable of mining (and not just mining - also powering large participating nodes!) from more manufacturers is the best way I can think of to prevent centralized control. In a perfect world every toaster would brown the bread with a few hundred watts of hashes.

The way I see it the next generation of GPUs is likely to represent a major shift in capabilities much larger than previous years, due to GDDR6 and other factors. This will force an upgrade-or-die world on all existing GPU miners over time. The best possible scenario is a variety of competitive upgrade paths that spread out hash power and PoW amongst multiple technologies and manufacturers.