A straight hash has the disadvantage that everyone who grabs the browser is likely to use the same dns provider for a given site. A hash ^ my machine UUID or MAC solves that.
That said, DoH only centralizes DNS until it is widely adopted enough. I think the ‘encrypt everything’ push at the moment has enough momentum to actually get DoH or similar specs implemented in enough places to have a chance of not centralizing. That does require a bit of a gamble on our parts, but it is a step in the right direction.