I don't think tether has such big power to take Bitcoin down. Stable coin is not enough and if people are going to those fiat-based, I'm pretty sure there will be more fiat-based tokens and crypto space is nothing different with fiat space.
If a stable coin takes place of bitcoin in future then it literally meant to be the end of crypto era,so people who had thing though need to think why they need stable coins which is centralized and against the decentralized payment system.IMO,stable coins will not survive too long,they have high hope until the coins were traded when crypto comes for payment then exchanges and stable coins will lose its demand.