Question: Since each #Bitcoin transaction includes a timestamp known to all, what practical power would a majority miner even have?

Answer: Mining is how the network determines the ordering of transactions, not by timestamps. A timestamp can be forged trivially (it’s just a number) but producing a block with sufficient Proof-of-Work is costly.

A majority miner has the power to change the order of transactions by producing alternative blocks, enabling the miner to erase previously included transactions from the ledger. This power can be used to defraud recipients or censor transactions arbitrarily.