[dmm]

You're right, a non-mining node still enforces the consensus rules. A non-mining node will reject any invalid blocks. Even if I controlled 100% of the mining hashrate I could not create a block that did something to violate the consensus rules. For example I couldn't mine a block that resulted in >21MM bitcoins.

[sobani]

You can definitely 'mine' such a block. The problem is getting the rest of the network to accept it. In fact, with Bitcoin Cash you can basically see what happens if blocks are created according to different rules.

However I image if suddenl 100% of the miners decided 42MM bitcoins is better (because 42), something a lot of users would be opposed to (printing money), things would become 'interesting'.