The way I see it, miners will mine Currency A until it becomes too difficult, and due to the market size, there isn't enough transaction fees to sustain mining. So bulk of them then switch to Currency B. Now nobody can use Currency A because difficulty A is too high for blocks to get processed efficiently and time to next correction is 2 years down the road. Ditto Currency B and new Currency C, ad nauseum.
This isn't a huge problem for two reasons:
1) The people who hold a currency will be incentivized to mine it at least enough to get the difficulty back down.
2) Alternative currencies can have any rules they want for when and how difficulty is adjusted.
The alternative currencies, at least for now, are not particularly likely to inflate the supply of bitcoins anywhere near as much as they inflate the demand. Any additional attention placed on virtual peer-to-peer currencies will spillover into more interest in bitcoins.