I've realized a new dilemma.

Assuming no transaction demand and no reason to hold the coin other than speculative investment demand, then the altcoin has to be appreciating faster than Bitcoin, else there will be a pernicious downward spiral. Litecoin may be caught in that now.

This realization seems to indicate there can only be one coin that survives, unless there is some reason to hold the coin other than its relative rate of appreciation.

One feature that might qualify is strong anonymity. Someone might resist transferring a balance to BTC given BTC's weaker anonymity, if anonymity was required for the holder.