Let's discuss the potential effects of stable coins which are bound to usd or eur. We usually use them to avoid the volatility or short bitcoin in a way. There are many stable coins in the last year. By writing threat, I didn't mean issues like tether's issue. I mean, they are not coins, they are just usd and eur.
Only stable coins are going to be threat to other stable coins, simply as that. I don't think that bitcoin will be affected negatively in any way. If I'm not mistaken though, there are rumours that Tether are being used to manipulate the price of bitcoin as early as 2017, but we all know that crypto market is always based on news and speculation so I don't know if this is true or not.
And maybe that is the closest argument you can say about stable coins being a threat, using them as a sort of tool to manipulate the price to favour whales or big entities from behind.