Yup, that's why there is a 75% threshold so your scenario won't happen. 75% is more than enough. It means on the worst case, 75% agreed to leave at most 25% behind. I doubt 25% will stick with the old chain anyway.
I guess you didn't closely follow about what happened with NotXT.
I can guarantee you that there will be fake BitcoinClassic nodes created so that it seems that 75% is achieved and then after one day - that 75% will disappear (and this is why all exchanges will simply stop exchanging as they know that this will happen).