So, there's namecoin, essentially the first special-purpose cryptocurrency. One might suppose that other such special-purpose cryptocurrencies appear over time.
Then we'll have Bitcoin, which will be used as a currency and for some applications, such as domain-name registration will be exchanged into another cryptocurrency. The other cryptocurrencies are all special-purpose, thus their value is much lower than that of the Bitcoin, but they might become an impotant part in an cryptocurrency ecosystem, and people will start to rely on them. However there will be much less mining power securing them. An attacker could thus accumulate some computing power equivalent to the current mining power of any of these special-purpose cryptocurrencies, and destroy them one by one. Essentially dealing a devastating blow to the cryptocurrency ecosystrem without ever attacking Bitcoin itslef directly.
Is such a scenario realistic? What would be the best defense?