first paragraph instills FEAR, which is unwarranted.
so make
"ever obtains more than 50% of all hashing power for a proof-of-work coin they can cause significant damage to the blockchain."
which sounds like simply getting >50% causes the damage
to
ever obtains more than 50% of all hashing power for a proof-of-work coin they could cause significant damage to the blockchain, if their lack of ethics deemed so.
next..
Preventing Confirmations
Mining with the majority of hashing power virtually guarantees that you will be the first to successfully hash every block more quickly than the rest of the network. If a 50+% miner wants to prevent certain transactions from being added to a block and confirmed on the blockchain, all they need to do is complete blocks, without the unwanted transaction in them, faster than the rest of the network. You really don't need to worry about these transactions, though. They will simply sit with no confirmations until the miners hash rate drops to below 50% and they lose control of the network.
even without 50%, denying transactions IS happening right now. as miners are greedy and only let a few hundred out of the 4k transaction limit, and mainly only the ones with fee's attached.. so that whole paragraph has no below 50% barrier as a prevention method