Even if the volume of Tether overtook bitcoin, we still can't call it the preferred crypto. Though there is a small number of people that are attracted by the interest provided in the stablecoin no trader or investor would prefer to keep their money in stablecoin. If they were, they wouldn't be exchanging it from fiat.
Tether might be the universal stablecoin at the moment but they are slowly losing their grounds with the introduction of new stablecoins.