Might be a silly question, but what has to happen for them to get $100 billion?

Notional is actually $87bn as some contracts have expired. They pay out 1% of notional for every 1% that CPI is below the strike CPI level on the day of expiry. So for them to be worth $87bn, the price of everything in the CPI basket has to go to zero in each relevant territory. Not zero growth, zero absolute. If that happens, I’m going shopping.

Given average expiry is 2.9 years away, and they’re not currently in the money, you’ve got to have quite a rapid deflation for them to be worth $1bn at expiry, let alone $100bn.

That said, they can be sold to a greater fool, if one can be found who will pay a significant sum. And 2.9 is the average, so some of the contracts may be significantly longer. But I’d be very wary of pencilling in a best case valuation greater than a few hundred million.