A successful 51% attack (PoW or PoS) would allow the attacker to double spend any token, not just ether, so the greatest possible payoff is equal to the value of all ERC20-type tokens plus all ether. But since the only token used for mining/staking is ether, the value of the reward for doing so isn't affected by the value of the ERC20-type tokens. So if the price of ether goes up, the value of mining/staking rewards will too, but the same isn't true for ERC20-type tokens. It seems like the more valuable ERC20-type tokens are relative to ether, the more profitable an attack is and the less stable ethereum is.
Am I right about this? And if so, is this is a known issue?