For reference, I am the author of Easy Passwords extension which uses a very similar concept.
Your concerns are valid of course but not necessarily for PBKDF2 which is used here. As long as a significantly high number of iterations is used bruteforcing any non-trivial master password from a derived one would take so long that it becomes unrealistic. Of course, ideally you should pick a strong password as your master password (Easy Passwords actively encourages that), it would also be recommendable with an encrypted key store. LessPass uses merely 8192 iterations however, this is way too low - recommendations vary but I would consider anything below 100k insecure these days. But with a sufficient number of iterations and a strong master password your biggest worry by far should be a malware infestation on your computer - both with this approach and with an encrypted key storage.
Your concerns are valid of course but not necessarily for PBKDF2 which is used here. As long as a significantly high number of iterations is used bruteforcing any non-trivial master password from a derived one would take so long that it becomes unrealistic. Of course, ideally you should pick a strong password as your master password (Easy Passwords actively encourages that), it would also be recommendable with an encrypted key store. LessPass uses merely 8192 iterations however, this is way too low - recommendations vary but I would consider anything below 100k insecure these days. But with a sufficient number of iterations and a strong master password your biggest worry by far should be a malware infestation on your computer - both with this approach and with an encrypted key storage.