Now this is powerful!

That’s not really true. Most third parties are insured against hacks and make good any loss, which is why they can charge a fee for custody.

A single policy change is unlikely to be legal if it ‘wipes out’ people. As in they would be sued. If it’s a government policy change that wipes people out, like state confiscation, well you’d likely see that coming through parliament well in advance in the West. In some dictatorship then sure it’s a risk.