[...] where as many as 20X this number of CPUs are already in use for other altcoins. [...]
Could you kindly share where do you get this figure from?
People, are constantly doing calculations for 51% attacks, they are posted all over the place.
Just google it, if you are that interested in it, rather than being lazy and asking someone else to do your own work.
And, at least 2 of the tiny coins (minimal-like cash startups) imitating the Ethereum Foundation's algorithm open source information, have had successful 51% attacks on them - so, yes it does happen when the hashrate is really tiny on the Dagger algorithm.
However, I'd reckon that direct clone coin of Ethereum, does not have to worry about a Classic 51% attack
