If it takes you 20 Petahash to find a block every 10 minutes and you remove 15 Petahash it will take you 40 minutes to find a block until the difficulty changes. My guess is that after the 2 months the difficulty will swing the other way an they will have 5-7 min block times. The question is do the miners have the extra money sitting around to mine for 2 months while only getting 25% of the payout they should be getting. Next the value of Corecoin at this point should be very low as most Bitcoin users do not care about the blocksize debate. What they do care about is is paying the lowest fees they can, store of value, and fast transactions. These 40 minute block times should last about 8 weeks if they can maintain 25% hashing power. Slower blocks and replay attacks would only happen to people trying to maintain a minority chain. You forgot also the minority chain would be 75% less secure. Forget the politics for just and second and consider, as a miner what chain would you mine? As a Bitcoin holder using it as a store of value or for commerce, what chain would you rather have your coins on.