Of course, there are no threats to the existence of bitcoin from stable coins, but stable coins, in my opinion, have a definite negative and positive impact on bitcoin.
Stable coins attract the attention of new people, which will inevitably increase the popularity of the entire decentralized cryptocurrency. However, stable coins play the role of a means of payment, they are more convenient in calculations than bitcoin, and therefore some people will use stable coins instead of bitcoin as a means of payment.