Well it's happening, so that's our brave new world.
If a currency is weak in some way then people won't be interested in it.
Bitcoins difficulty can go down as well as up - if no one is mining bitcoin then it will become easier to mine bitcoin thus attracting miners, who will raise the difficulty again (that's exactly what happened this month - the next difficulty is less than the current difficulty).
Difficulty 1,890,362
Next Difficulty 1,868,794
http://www.bitcoinwatch.com/The problem is the time till difficulty correction. Just look at namecoin, the current estimate is Nov before correction, a total of 5 months. Instead of 6 blocks per hour, it's averaged less than 1 per hour. If people start getting tired of long wait time for transactions to be confirmed, then switch to Currency B, it will make the time to next correction longer and longer and eventually currency A will just die.
If there isn't some kind of sensible control/selection, the whole crypto-currency movement will simply go into an infinite loop and eventually dying off as people get sick of the same old shit again and again.