It can be bought, in fact by anybody, including a potential attacker. The attacker could even be a group of miners on the current chain. How much would Blackrock be willing to spend to secure their new chain? Even if they did that, would the new chain retain the same value proposition if the code changes were nefarious or closed source? Reminds me of the hash wars, except the network is an order of magnitude more secure than it was during the hash wars.