If you realize that point, then the question is moot.  Bitcoin derives a lot of its popularity from the fact that its value is subject to the free market, meaning that its price moves according to supply and demand.  Stable coins don't do that, and there is never going to be significant interest in them other than using them as a substitute for cash on exchanges.

So no, there isn't a stable coin in existence or one to be yet created that will present a threat to bitcoin, or to any altcoins.