Whether it negates the benefits of 2FA (i.e. whether it's 1FA) doesn't depend on the threat model. The threat model is what makes an individual decide whether 2FA is worth it
- "an attacker still only requires a password (which may be brute forced" Brute force? Then it wasn't generated securely. Use Keepass' built-in generator or another one that is meant to generate passwords with. You could make the same argument for the 2FA seed: that can also be generated insecurely or set manually by hand
- "or otherwise obtained from badly secured websites" The attackers can obtain the 2FA seed in the same way. Furthermore, the 2FA seed is stored plaintext in the website's database whereas the password is typically stored in hashed form (which is unbruteforceable if you generated it securely)
- "they _also_ need a copy of your keepass-file, the [main] password" The attacker either got the website's password from this same file (so they evidently have that file and the main password to unlock it), or if you imagine a scenario where the site got hacked and the stupid site stored your password in plain text, then they have access to the site (and the 2FA seed) anyway because they hacked it. So long as you don't re-use passwords (don't need 2FA for that, just a password manager), a site's breach has no impact beyond this one site that was hacked outside of your control
- "and any other factor to _it_" Do you mean the key file here? Or what other factor? Either way, again: if they need that to open the vault then they can't get at the passwords either, so what benefit does {2FA when stored inside the password vault} have?
I don't know of a scenario where an attacker can get the website's password from the vault, but not the website's 2FA seed, when you store both inside that same (encrypted) file
While it seems a reasonable assumption that 2F seed and password are stored alongside eachother, the vuln does not need to expose both. We don't know. But we do know more factors means more opportunity for 1 factor to be accidentally safe.
