I hope that they get 75% miner support right away so that I can watch their miners saying, "Oh shit, we just mined a ton of worthless altcoins!".
Why are you so sure it will be an altcoin? If 70% of the miners mine the new chain, they have the power to defrag the old chain without risk, like it happened so often with altcoins, and they also have an economic incentive to do so. No sane investor would buy the old coins, no miner would mine it, no merchant would accept it, no exchange would hold it, cause they are the most vulnerable altcoin on earth, just living on bless of the miner's of the new bitcoin.
If the fork happens like described by classic / xt, it will play out very soon. People that try to double spent between the chains will loose their coins, people who kept their coins unspent on the old chain can simple take them to the new chain.
If I think about it ... the miner's of the new bitcoin can just 51 the old coin and force it into their fork. After this there will be some old nodes left, that can still spent, but not receive nor relay ... it's really easy. A fork with the supermajority of miners will be the only one.