Actually, he hasn’t deleted the 50 BC. They are still there but they are trapped. He only deleted the only copy of the key to transfer them, so those BC will be trapped there “forever”.

Actually it’s possible to brute-force search for the key and make the BC free, and transfer them. But with the same method it’s possible to steal BC. The brute-force method is just too slow, and would require a few thousands or perhaps millions of years. (I don’t remember the exact estimation.)

With the help of the Moor’s law and some advance breaking the cryptographic security measures, this can happen sooner, but this would be a problem for every BC user.