Says who? Nouriel Roubini, Dr. Doom? He's also been saying for years that Bitcoin is headed to $0.
Nocoiners always begin their analysis from the conclusion that Bitcoin will collapse; the only thing that changes is the future cause. First it was regulatory crackdowns because of illegal activity. Then it was the Mt. Gox Willy Bot, and how it had fooled everyone into buying too high. Then it was Tether.....
I don't know exactly what a Tether collapse would look like, but I am definitely not convinced the devaluation of one of Bitcoin's primary base pairs (USDT) would lead to Bitcoin's demise. Maybe some short term chaos and volatility, but I really don't understand why people freak out about this.
What do you even mean by "systemic risk" anyway?