Just looking at vanitygen-plus (currently supporting almost one-hundred crypto-currencies) do its stuff, and it is capable of over 548,990 keys/sec on my CPU alone.
Well developed ASIC is several orders of magnitude faster than my CPU, wouldn't it be profitable for a company with the resources to do so to develop such a key miner, even if it is a one-off?
The reward (actually, theft) is access to the private keys of any address. Surely the development costs are not so prohibitive for ASIC as they are for general-purpose CPU's?
Future developments in this area seem like the most realistic threat to crypto-currency security so far.