When miners mine, if they don't make a pool, the miners are definitely competing each other. I believe most miner use open source program. The nonce they choose all depend on same principle, that means they all compete the difficulty redundantly, in most cases only the best Miner or miner pool wins.So if one wants to fraud, he can just set up another miner pool that is better than the integrate best, then the system crashes. That means the frauder doesn't need to be better than the sum of 51% whole Bitcoin system.

The reason why no body break this is because we still have something to mine,so if I have this super mining system, I would mine instead of crash the system. But when left bitcoins are so little, and Bitcoin only face two destiny, either raise the transaction fee greatly or some big miner may go rogue to cash out their money as much as possible. Either way will ruin Bitcoin eventually.

if miners are competing each other, actually a frauder doesn't need to have more computational power than all the integrate nodes, he only needs to have more computational power than the best of the integrate ones.
And more, the system can not be used for large amount of transactions, if one transaction involves billions of usd, I may prepare a huge farm of computer pool, to revise the blockchain.