The primary market for tether has huge transaction fees, so any collapse will happen on secondary first.

I am not a markets expert, but I would guess that straying far from $1.000 on the secondary markets for any length of time at all is a sign that something very bad is simmering under the surface. There should, I would think, be enough liquidity in a fungible, theoretically stable asset like this that such a glaring arbitrage opportunity can only exist on quantum time scales.