I have a bit of SPXL, a 3x leveraged ETF. Suppose if I sit on it long-term. Theoretically it could go to 0, if the underlying index declined by >33.3% in one day (before the rebalancing could save it).

In practice this seems unlikely -- we'd expect the exchanges to use a circuit-breaker, or do some sort of rollback.

...Right?

Will I lose the money? Resolves YES if my SPXL position goes to 0, resolves NO in 2040-Jan if this doesn't happen.