So, that's an interesting pair of ideas, in that it gets me thinking that any Bitcoin-style cryptocurrency might ultimately be doomed by its own design.
Shooting from the hip:
They've go this 51% vulnerability that is well known and hypothetically cannot be truly closed. Instead, we rely on the idea that mounting such an attack would be "too expensive". But at the same time, the cost/benefit of mounting such an attack is fairly easy to estimate using public data - all you really need to know is the cost to get to 51% and stay there for a given amount of time, which you can infer by monitoring mining activity, and the current price of the currency you'd want to attack. And you have to assume that whenever the cost of mounting such an attack dips below the benefit, such a thing _will_ happen.
So then, I think that implies that the only other feature you'd need to throw into the mix to ensure a cryptocurrency is ultimately doomed is to make the rate at which new coin can be mined asymptotically approach zero. Such a feature would mean that, in the long term, miners' revenue would ultimately be dominated by transaction fees, which, this being a supremely commodity service, market forces will presumably tend to keep relatively low. That would, in turn, limit the number of miners the economy can support, which would serve to limit the cost of mounting a 51% attack.
Meanwhile, what with a money supply that can't grow being inherently deflationary, the benefit of mounting such an attack would be constantly growing, for as long as said cryptocurrency remains in use.