Do we know that this is the only back door? We can't be certain until we read the actual text of the agreement. Everything else is speculation. Informed speculation, perhaps, but still speculation.
Even if this is the only back door, it still leaves the door open for endless copyright extension by the US. I expect a bill rider on some critical piece of legislation in late 2018, (around midnight, on a voice vote).
If you look at Pdurrant's bet thread, I have a 1/2 oz of gold riding on this. . . (And that is only that there will be an <attempt> to extend it, not that it would be successful.)