increase the risk that diggers will jointly perform attacks to make up 51% of control. Transactional activities or code changes in the blockchain to their will.
Apart from the miners controlling the hash-rate, which is basically the security behind the protocol; there are full nodes that are supposed to validate any new transactions and blocks.
If you run a full node then it is only going to accept those blocks which fulfill the requirements of the code as defined by the bitcoin core client. Any code changes done by miners will not be accepted by these full nodes as valid and will not be propagated or included into the mempool.