Well if it suddenly would (for some reason) fail, it would shock the markets because there would be ton of liquidity problems to deal with. It would be chaos all around, and of course that would have short time effects on bitcoin price as well.

I respect Vitalik as an intellectual but i don't have a clue why he thinks Tether would have any problems, Tether are closely working with regulators unlike nearly every IDO project eth has btw.