There are many practical reasons what this attack won't work
The attack assumes that "the attacker needs to be able to afford to purchase 50% of the votecoins". The problem is that affordability is undecidable and highly unprobable due to the way markets are working.
Liquidity: markets don't allow to instantly and atomically execute huge positions: they are limited by liquidity and I have never seen a market with 50% of the float available for sale.
Reflexivity: markets tend to trend against large interests entering the market. Executing too quick will make the price grow superlinearly. Executing too slowly allows observers to do trend analysis and detect the buy pressure. Either way, there is no way to buy 50% of the float of a market without sending the price through the roof, which makes "affordability" something undecidable in advance and highly doubtful.
Fixed income cost: TruthCoin yield commissions. As such, there is an incentive for holders to hold them in the network as opposed to an exchange. This will further reduce liquidity. And if there are exchanges that will offer to hold the coins, participate in voting, and pay back the yield to despositors, this will foil the attack further as now one or serveral exchanges will have to risk their whole business to gain from this attack and it's a safe bet that they won't.
Biaised equilibrium: the affordability condition ties the voting game to another game: the price disovery mecanism of the market. If you publish a contract that will in fine oblige you to make large TruthCoin purchases on the market, you are giving a strong bullish signal to market participants (the voters themselves) that their TruthCoins are going to gain value in the event where the attack fails, which means that they will move their orders to sell higher, and possibly even buy into the orders of less responsive participants, so that the attack will increasingly be perceived as less and less affordable, the contract as more and more liquely to default, which will desincentivize voters to put all the truthcoins on the fake outcome since the contract and will incentivize them to simply withdraw their coins to the exchange and try to sell them in the crazy rally the contract is going to generate. I don't have a payoff matrix as the situation is near to impossible to model, but the least I can say is that in that dual-attractor setup with a market feeding into a prisonner-dilemma game, there is no trivial winning strategy for the attacker and all participants. The best strategy for the attacker is to not attack.
Now you could say that this can be worked around by buying progressively enough TruthCoins on the market before the attack. But then the attacker will own the majority of the shares, so he doesn't even need the contract, he can just chose the outcome he likes, win his bet, and watch the value of all his CashCoin and TruthCoin holdings go away in smoke as the market realizes that TruthCoin has been attacked and makes the price tank. And of course, no time to close so large positions in the market before the price goes south.
Either way, that attack is completely unrealistic.
The attack assumes that "the attacker needs to be able to afford to purchase 50% of the votecoins". The problem is that affordability is undecidable and highly unprobable due to the way markets are working.
Liquidity: markets don't allow to instantly and atomically execute huge positions: they are limited by liquidity and I have never seen a market with 50% of the float available for sale.
Reflexivity: markets tend to trend against large interests entering the market. Executing too quick will make the price grow superlinearly. Executing too slowly allows observers to do trend analysis and detect the buy pressure. Either way, there is no way to buy 50% of the float of a market without sending the price through the roof, which makes "affordability" something undecidable in advance and highly doubtful.
Fixed income cost: TruthCoin yield commissions. As such, there is an incentive for holders to hold them in the network as opposed to an exchange. This will further reduce liquidity. And if there are exchanges that will offer to hold the coins, participate in voting, and pay back the yield to despositors, this will foil the attack further as now one or serveral exchanges will have to risk their whole business to gain from this attack and it's a safe bet that they won't.
Biaised equilibrium: the affordability condition ties the voting game to another game: the price disovery mecanism of the market. If you publish a contract that will in fine oblige you to make large TruthCoin purchases on the market, you are giving a strong bullish signal to market participants (the voters themselves) that their TruthCoins are going to gain value in the event where the attack fails, which means that they will move their orders to sell higher, and possibly even buy into the orders of less responsive participants, so that the attack will increasingly be perceived as less and less affordable, the contract as more and more liquely to default, which will desincentivize voters to put all the truthcoins on the fake outcome since the contract and will incentivize them to simply withdraw their coins to the exchange and try to sell them in the crazy rally the contract is going to generate. I don't have a payoff matrix as the situation is near to impossible to model, but the least I can say is that in that dual-attractor setup with a market feeding into a prisonner-dilemma game, there is no trivial winning strategy for the attacker and all participants. The best strategy for the attacker is to not attack.
Now you could say that this can be worked around by buying progressively enough TruthCoins on the market before the attack. But then the attacker will own the majority of the shares, so he doesn't even need the contract, he can just chose the outcome he likes, win his bet, and watch the value of all his CashCoin and TruthCoin holdings go away in smoke as the market realizes that TruthCoin has been attacked and makes the price tank. And of course, no time to close so large positions in the market before the price goes south.
Either way, that attack is completely unrealistic.
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