I think everyone should be using passphrases, since they are the best way to provide plausible deniability to your wallets, and also provide extra security should an attacker discover your seed phrase back up. However, they do absolutely nothing to prevent someone from brute forcing or stumbling across one of your private keys (which is already so rare as to essentially be impossible before the the death of the sun).

The security of a bitcoin private key is 128 bits. It doesn't matter if you add an entire paragraph or 10,000 random characters to your seed phrase - your private keys will still have a security of 128 bits. Further, given the way in which private keys are generated from a seed phrase, there is just as much chance as a completely different seed phrase generating an address which is the same as an address from your wallet, with or without an additional passphrase. In fact, I would say this is more likely, since many wallets will generate a single seed phrase, but then generate 20+ addresses from that single seed phrase, meaning that there are 20x as many chances of an address being duplicated than of a seed phrase being duplicated.