That is a good point, but consider the following: 1. If a cryptocurrency is dominated by specialized miners, then you have a centralization effect as the major

That is a good point, but consider the following: 1. If a cryptocurrency is dominated by specialized miners, then you have a centralization effect as the majority hash-rate can be more controlled by monopolies that can take advantage of their economies of scale. Because mining is mostly done by companies at scale, it is easier for governments to impose regulations on miners and control the type of transactions that are allowed to be verified. 2. Vendors accepting cryptocurrency can always choose their own level of risk by deciding how many blocks to wait before accepting a transaction. This allows them to "wait out" temporary attacks. I would say that the permanent miner takeover as described #1 is an order of magnitude more threatening than the temporary attacks as you've described. But it is still quite an issue. Specialized hardware will depreciate if it is tied to a cryptocurrency that has been attacked. So miners are incentivized not to attack, lest they devalue the resell value of their hardware. Note that this advantage doesn't really apply to GPU-mined cryptocurrencies.