As you can see in this famous XKCD comic, 44 bits of entropy, that is to say 4 words of the 2000 most common words in english (like for bip39 seeds), reprents 550 years of computing at 1000results/s so I doubt some hackers have already try to compute and check all those addresses, and I'm not even talking about the ability of monitoring all of them...
That's a different scenario, though. That's someone trying to brute force access to an account login, not generate private keys. With the right hardware, programs such as Vanity Search can generate in excess of a billion keys per second (See here for benchmarks: https://bitcointalksearch.org/topic/m.50823897). 44 bits of entropy can now be broken in around 4 hours. Bear in mind this only has to be done once, and the attacker can simply then generate a look-up table of public addresses. As soon as one of those public addresses is used, he already knows the private key and can automatically generate a transaction to steal the funds. In fact, that's exactly what one of them was doing years ago: https://news.ycombinator.com/item?id=7368283And this is only considering a single attacker. We know there are many of these attackers out there. If you read through the other thread I linked to before, in some of the cases of a brain wallet being used, there were 4 or 5 different transactions generated within a second all trying to steal the funds to different addresses.
