I do not think that it would be a problem, market cappitalization doesn´t mean that such amount of money is put into the Ethereum. Market cappitalization is calculated on the price and number of tokens. And if you think about it, Tether wouldn´t fight with Ethereum because Tether can´t work without the Ethereum network. Of course that they can move into other chain, but Ethereum has the most active users and it is user friendly.