But, how would the wealth be transfered to a new system and what would be the exchange rate?
Wealth would not have to be transferred per say. Rather, a new, cryptographically sound system would be adopted, and previous owners would be given new currency in proportion to their old balance of BC. The exchange rate would be determined solely by the market's usage of the new currency.
Imagine a verified public registry that voluntarily ties BC addresses to email addresses (by signing your email address with your private key for the BC address). If BC failed, the new system would take a certain historical block from BC at a fraudless time. Then, supposing again that 21 million units of new currency were being generated, and that 80% of the old BC currency had been registered, then each time a node generates a block for the new currency they would get to keep 20% of some number of new currency while every registered address would receive some tiny share of the other 80% in proportion to their registered BC at their registered email address. When all new currency generation is done, the owners of the 80% of registered BC money will have 80% of the new money, while the lost 20% is reissued.
I would bet that if BC had seen widespread adoption, and if many people had registered their addresses, then the new currency would be readily accepted because so many people would be getting it for free and in their "fair" amount.
Perhaps this could be combined with an insurance plan that had a predetermined and guaranteed payout written in the policy?
That's another interesting idea. Basically, some individual could choose to setup their own insurance company for BC where people pay a certain percentage of their protected BC amount. This company would then keep a registry similar to that above, but only for its clients (It's necessary to verify that clients have the BC amount they claim). Instead of issuing new currency in case of catastrophic failure, they could simply issue USD's. Again, the biggest challenge would be setting clear terms for when payout would occur and which block would be treated as the truth. Perhaps they could pay out from the last block issued before a mathematical proof of the system's unsoundness was offered?