51% of mining power already has defacto control of the network.
Wrong that is not 'control'. They can attempt to attack the network, after which they would get 'fired' with a algorithm change.
Therefore, BU doesn't actually "give miners more power", it just facilitates consensus.
BU breaks consensus and attempts to change the security model into something that isn't even adequately tested.
BU facilitates breaking consensus, not enforcing it. That's the whole point of BU.
No-one will be able to trust the validity of on-chain transactions, as BU can fork ior reorg tself unexpectedly at any time. The ability to alter the consensus rules on the fly turns forks and re-orgs into a feature, when they're highly undesirable for a stable uniform blockchain.
Indeed.