I think it is legitimate to fork Bitcoin with 51%, it might be ill advised but it is not wrong, since this represents the freedom and will of the participants of the Bitcoin network. Nobody is forcing the miners to do anything, they have the free choice to point their hash power wherever they want. Bitcoin is a type of democracy where the economic majority rules. However unlike democratic states, tyranny of the majority is impossible under the consensus mechanism of Bitcoin because the minority of such a split can choose to stay with the old rules in effect splitting Bitcoin in two. This is a good thing and a beautiful solution to the age old problem of tyranny of the majority. This is why a temporary majority can not force its will on the minority as you claim.

I know what it means, you know consensus did exist before Bitcoin. Do me a favor and learn more about political philosophy and history. Then you would realize that consensus or unanimity is impossible among large groups of people. Which is why we need to build these political structures in order to best approximate effective decision making processes. Within Bitcoin this mechanism exists as the consensus mechanism, it is called consensus because most of the time there is unanimity however when fork events happen because of disagreement, then unanimity becomes impossible, it can then only be resolved through a fork. This is the decision making process of Bitcoin to think that we can all just agree with each other is completely ignoring human nature and the lessons taught by history.