The human aspect. What will happen (any blocksize really) if a powerful opponent fills the queue with spam transactions with high fee, like 0.01 BTC or even higher? A few million dollars could suffocate the functioning of bitcoin for months. My guess: As such situation is not advantageous for the miners despite the good fee money, they would find the spam transactions using heuristics, then fill blocks say half with spam transaction to take the good money, and half with normal transactions. The attack would be costly for the attacker, reduce the payment functionality to a degree, otherwise bitcoin would continue as always.