blake2b already has asics. thats why it was picked. and there is already hashrate available for rent in many places.

and bitmain cant sell their blake2b coins for usd easily, there is no exchange, (replayablility is kinda protecting us). so there is little chance they can sustain and waste resources on multiple asics. they earn nothing. attacking corechain is profitable. because they get paid for it. blake2b is not easy to attack without wasting energy for no return.

also if they attack we can change pow algo again, that's something we do now, and we proved that we can do it, unlike the other chain.