If mining continues on the original chain, there will be two coins — Bitcoin (original chain) and ClassicCoin (big blocks / hard fork chain). The original chain can persist regardless of how much more hashing capacity mines on BItcoin Classic.
These two coins likely will differ greatly in price. Perhaps a Bitcoin will trade at $100 and a ClassicCoin will trade at $250.
But what if at the time of the fork you had bitcoins deposited at an exchange? How will your exchange handle this situation?
Does your exchange’s terms of service (ToS) address this situation? If not, might your exchange just claim "we use Bitcoin Classic and follow the chain with the greatest work" and use that as justification to steal your $100 bitcoin?
There are arguments that they legally wouldn’t be able to do that: https://www.reddit.com/r/Bitcoin/comments/41j4j1/i_dont_care_about_the_upcoming_fork_but_i_dont/cz2sgvd?context=3
But are they prepared (or preparing) to support handling of both coins? Not a single exchange has mentioned this as something they are working on, or planning on even!
Perhaps everyone thinks that this is something that can be addressed later. But once Bitcoin Classic is released, the hard fork can come as soon as 5 weeks later! If blocks are full already, how will the Bitcoin blockchain handle all the withdrawal requests once people realize they should pull their coins from exchanges and hold the private keys themselves?
If this added transaction volume causes a fee market to develop, will the withdrawal transactions from the exchanges (who generally pay the fee for withdrawals) get confirmed even? Or will those transactions instead be left on the sideline while other user’s transactions where the appropriate fee was paid get included instead?tl:dr: The exits are extremely narrow. The time to get your bitcoins off the exchanges is NOW!
