But, doesn't a DB compromise mean that the attacker would have the TOTP seed as well? It can only increase your account security elsewhere, but also not re-using password prevents the IA leak from hurting you elsewhere as well?
Note I'm quoting HIBP's advice from the email they've sent me! I'm absolutely not recommending to store one's 2FA secrets in the same place as the password!
Even if one uses 2FA for the password manager, it stops proving "something you have" in addition to something you know and you're one unlock away from malware vacuuming it all up. The point of 2FA is to be on a separate device you need to have on hand
Of course, the same logic goes for a password manager in the first place, but password reuse is a big enough problem that (for most people's threat model) it seems to be a net positive. 2FA tokens don't have that reuse issue