How centralized is Bitcoin mining regarding the providers of the hardware? Do people care when buying hardware?

People speculate a lot about how much money it would take to arm a 51% attack but rarely do I see people worry about that money coming from trusting customers.

The currently biggest producer of Bitcoin mining hardware - BitMain - set precedence for a kill switch https://bitcointalk.org/index.php?topic=1888573.0 that if present could be used to "incentivize" miners to follow their policy decisions - honor blacklists for example.

How well are we protected against miners activating such a kill switch after some specific activation date? I assume they are closed source, so it would be hard to detect before a triggering date.