I don't know why people think that stable coins are a threat to bitcoin. Infact they are playing a bigger role to helping to solve the problem of high volatility in the cryptospace that has left most traders with losses.
The threat is in the potential fallout from the companies behind these coins printing too much fake money and going bankrupt. USDt was supposed to be backed 1:1 by usd. People were asking them to show them proof that it's backed and got none. Finally the company admitted that some of the USDt in circulation is not backed. Who wants to be the last fool to turn their USDt into USD? Not me.