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.
It is worth remembering that the larger the capitalization of the USDT, the less interest is guaranteed in cash dollars. Therefore, the larger the capitalization, the more risky this asset becomes.