Seriously though, what are the odds that someone has been quietly spending 10s/100s of millions in cloud compute to brute force the keys for old wallets?
you are correct at first pass, but it's a fact wallets have been cracked many times, perhaps at least 100s of millions of dollars. The "keyspace" for the cracked wallet is a subset of the nominal keyspace - the much smaller space covered by either a flawed random number generator (RNG), or the whole brainwallet fiasco, or a RNG where a seed is crackable (e.g. milliseconds since 1970 or unix epoch - some cracks, whitehat, have used this method). That's all what we know in the whitehat space, surely other tricks exist in the blackhat space