It does have at the moment at least it does have a fixed consensus mechanism. That said, we do eventually hope for even the consensus mechanism to become modular and potentially pluggable. The reason that that's not our priority right now is because A, we're really happy with our current consensus mechanism; B, there are subtleties to modularizing the consensus mechanism. Other projects are attempting it, Tezos being a notable one - we'll see how they got along.