Bitaddress uses randomness from different sources which would make it harder as you'll have to replicate both the tracked mouse movement as well as the randomness that was generated when the user enters the page.
These attacks can only work if they are using predictable variables as an entropy source. If and only if you can find a pattern in that generation, then you can reduce the search space significantly.
I'd argue that people should not be using PRNGs seeded with cryptographically secure entropy to make private keys especially on browsers in particular (which is the only method they have, they got no CSRNGs) because you're relying on the webpage to supply a good-enough entropy. Mouse and keyboard input that's made
during (not before) entropy gathering can also be tracked within the browser and webpage itself so all it takes is a malicious addon that tracks such movement and they can re-derive the entropy. When a PRNG is used this also allows them to make the private key too.