“Code is Law” fails again.
There will probably be a hardfork and increased moral hazard for auditing contract code. Why spend money finding bugs if the "immutable" blockchain can be rolled back?
Ethereum has had one particularly controversial fork, and so did Bitcoin. In both cases, both chains survived, with one significantly more popular than the other (in both hash rate, transactions, and market cap). (Un)fortunately, traditional law in real life does not permit two realities to coexist, so there the analogy falls apart somewhat.
Like, imagine if twenty members of congress kept their life savings in cash in uninsured briefcases and surprise, those briefcases were stolen. And they responded by passing emergency legislation invalidating the specific serial numbers stolen, and requiring every merchant and bank to screen for those serial numbers (and not those from any other e.g. bank robbery).
I would consider that kind of action to be very delegitimizing to the currency.
 I would consider it to be analogous to a fork when they e.g. deprecate an old design or migrate to the Euro.
I do think that calling blockchains "immutable" can be confusing to users not familiar with the concept and its nuances. And I personally hold no opinion on whether the currently locked up funds need to be returned to their owner with a fork. There is something to be said for both options, specifically in this case where there the "rightful owner" is clear (like the other cases mentioned in https://github.com/ethereum/EIPs/issues/156 ).
At the time of the DAO hack there also hadn’t been a public demonstration of the scale and danger of contract hacking. That’s also no longer true.
That said, ETH is an anarchist federation in the end, and the market will decide which reality is most valuable: the one in which the parity hack happened or the one in which it didn’t.
The Bamboo manifest:
There is a project under way to formalize Viper, which is the other newly developed programming language for Ethereum, as well: