Nodes have zero hashing power. Only economic power to the extent that they are paying miners with fees. The vast majority of nodes are economically irrelevant. Centralization is a threat on both the mining side and the merchant side.
https://github.com/libbitcoin/libbitcoin-system/wiki/Balance-of-Power-Fallacy
The majority of people running nodes are aware of the economics of bitcoin and support miners who contribute to the greater network security. If miners start acting in a hostile manner, I fully expect nodes to pivot and discontinue verifying transactions from rogue miners. This would make the whole 51% exercise an expensive and fruitless endeavor.
Node runners have a great deal of power which should not be overlooked.
Who's going to mine on the new chain?
The remaining 49% (or less) of the miners that proved themselves to be good actors
