How can you be so confident that the three pool operators wouldn't decide to attack together? They may have some secret relationship. I will ask again: Why do the three top pools account for EXACTLY 51% of the total hashrate?
I mean this in the nicest way possible. But it is stupid to think big companies that depend on bitcoin would do a 51 percent attack. It would destroy their future business which if FAR FAR greater then amount they make from one 51 percent attack.
The current biggest is antpool. Do you think they would risk sales on machines to pull an attack? Not to mention pool earnings?
Second is f2pool... do you think antpool and f2pool are best friends and work together on much other then block size recently?
And bitfury is 100 percent different as they are not counting on any others outside of them for hash.
How long have you been with bitcoin? Do you mine, if so how many T? I'm trying to guage why you are so in fear of this thing that will never happen.