If such scenario ever happens, good luck trying to convince the overwhelming majority that their version is not bitcoin.
Again, thats not how it works. You don't have to convince anyone of anything. Bitcoin is a protocol that is based on consensus, not who can pander to the most people. Frankly, this is really a no win situation. Both XT and Core have problems, rather than solving those problems, they are just fighting a publicity battle, which is causing a bigger issue. If Bitcoin works the way that people are trying to make it work, the way you suggest, it is the greatest security risk Bitcoin has ever had, and "Bitcoin" can be safely called dead unless its fixed. Block size issues need to be fixed, a solution that doesn't require 15Mb upload/download speeds to broadcast larger sized blocks needs to exist, anonymity things need to be added, amongst other things. But those can all wait, as soon as all it requires is a majority to modify Bitcoin, anyone who can buy support for whatever cause they want can, be that block size increases, block reward increase/decreases, etc.
Again, the protocol requires consensus. This isn't a majority rule system, because majority rule systems are not stable nor secure. While Theymos has expressed that he isn't a fan of XT's proposed changes, I believe his greater concern is the same I addressed a moment ago. The straight definition however of an Alt Coin, is a Blockchain/Coin that is not Bitcoin. Bitcoin requires 99.9% concensus. Without that concensus its not Bitcoin, and therefor an alt coin.
*99.9% is just an aproximation, whatever the actual value is is aproximate 100% - statistical misrepresentations/margin of error etc, but its near 100%.