So the 'truth' is whichever way the majority votes? Putting side the obvious issues with that, what prevents someone who has the largest warchest from sweeping

So the 'truth' is whichever way the majority votes? Putting side the obvious issues with that, what prevents someone who has the largest warchest from sweeping everything or is it just an assumption that no-one will have a 51% share blockchains are largely based on?