BU can even be configured to mimic the behavior of BIP100, BIP101, BIP102, BIP103 or BIP202. Or it can be setup to behave exactly the same as a Core node as well, the choice is yours, in different words it is up to the free market. Considering all of the possible options of who decides I would argue that this is the best solution.
It's not. It's up to the miners.
Stop promoting this misleading idea.
Consensus is an emergent property which flows from the will of the economic majority. Proof of work is the best way to measure this consensus. The pools act as proxy for the miners, pools behave in a similar way to representatives within a representative democracy. Then in turn the miners act as a proxy for the economic majority. Since the miners are incentivized to follow the economic majority. In effect the economic majority rules Bitcoin, in other words the market rules Bitcoin. Bitcoin relies on the economic self-interest of the masses to govern consensus.