Has this ever happened before?
Follow-up questions:
If there becomes 100 trillion wallets in use, do you think finding a wallet with a balance will become common? Is the current private key secure enough to last 10,000+ years?
Is there a way to scale up the security of the generated private keys somehow if needed someday?
This was already happened before, that many bitcoin wallets in blockchain has been hacked by someone and $100 million worth of bitcoin has been stolen, but, this hacking was not because of using bruteforce. Hacking a bitcoin wallet using its private key and using bruteforce to know this private key is impossible, maybe it is, if someone would develop this bruteforce up to something that can hack any kind of passwords. Private key contains a 64 character compose of numbers and letter, big and small letters, so it is impossible to do this thing.
I remember using bruteforce to hack someone's facebook and it failed, but sometime it works. Note, the facebook password is more weaker than the private key on every bitcoin wallet, that is why for me it is impossible to hack this by just using bruteforce.