I don't follow this stuff closely, so I'm clearly missing an important detail here: how can several different entities simultaneously have the ability to "easily" control 51% of the hashrate?
Imagine a new sport called zoccer. It's just like regular soccer, but if a team scores 5 goals within the 90 minutes, they automatically win the match.
Right now there are only a few small teams playing. But Nike has said they are willing to sponsor a tournament, if there is enough interest.
At any point, Barcelona or Manchester United or Real Madrid or Chelsea or AC Milan or any other large soccer team team can just say "we are now a zoccer club" and swoop in take Nike's money.
They all have the power to switch and dominate the new scene.