Tether is not a threat to Bitcoin.
Tether is a threat to Bitcoin price.

Since Tether is not properly backed, its demise will have the effect of a powerful earthquake over the whole crypto markets.
And the analogy with earthquake is not over: this will allow crypto markets rebuild stronger. So it's fine. Crypto markets rebuild fast and there are plenty of better stablecoins.

And like with any such natural phenomenon, you should not ask yourself if this will happen, you should ask when will it happen.