That's true theoretically and your argument still stands for a lot of cases.
However for practical purposes nodes only compute up to a certain amount of instructions. This is known as the "gas limit". Additionally it costs Ether to get them to run code so any DOS approached in this way is prohibitively expensive.
Due to the nature of mining on Ethereum (and most cryptocurrencies) you do not need the consent of many.
Miners work in pools, and it's up to the pool operators to make decisions for a hard fork. In the case of Ethereum it took about 4 or 5 mining pools to reach majority (over 51% of network hashing power).
A great deal of miners simply leave their mining rigs running and didn't make a decision either way. Miners largely do not care about any cryptocurrency. They will, understandably, mine whatever is most profitable.
Miners also depend heavily on ping times and the total mining power of their pools. With 15 second block times every millisecond counts to make sure you're the first with the right block and get your reward. There was really no time for anti-fork mining pools to get setup well and it takes months for a pool to amass an attractive amount of miner hashing power.
Overall the vote was about 5% for the fork, 1% against, and 94% abstaining. Ideally it would take a true 51% to change the network.
However for practical purposes nodes only compute up to a certain amount of instructions. This is known as the "gas limit". Additionally it costs Ether to get them to run code so any DOS approached in this way is prohibitively expensive.