The real threat 2fa auth solves is the fact people blame the site operator when they are hacked. 90% of the time it is due to reusing a password. The other 10% it is due to phishing. WebAuthn stops both. 2FA works not because it adds another factor, but because it removes choice from the user so they can't screw it up.
We used to think that
> It helps against an attack that's specifically targeting you
would be sufficient, because, like who cares about little old me? But in the era of API intelligence, setting up a specific agent to target every person seems automatable.
If all that was missing before was a sufficiently-motivated attacker who would learn about you and your patterns, and that couldn't scale because we didn't have enough dedicated bad guys, that seems like it's a bout to change pretty quick.
Even people that do this incorrectly and reuse passwords, probably also store their passwords in their browser, which is on the same device as their authenticator app. So I would guess by far majority of people are only using something they have twice.
Even if someone steals your phone, you should have a passcode. If they know or guess your passcode, well… someone could steal your house/car keys too, and we still carry them around anyway :)
In this scenario, you are assuming that the client is not compromised (since otherwise they would just steal it before you use it) the server is not compromised (otherwise what is the point), you do not have an active mitm (otherwise they could use the token directly instead of replaying).
All that really leaves is you have someone capable of passively eavesdropping a TLS connection (usually much harder to do than active mitm). I suppose someone literally looking over your shoulder or recording you with a hidden camera - but even then they just have to out-race you hitting submit.
Anyways. Not a realistic threat in my mind.
On the server side usually a "key" is stored, which for TOTP based 2fa would allow the attacker to create future 2fa tokens if they got ahold of the key. So what really saves you is the website choses the key not the user, meaning every website has a different one. Not the temporal nature.
Anyways, usual term for what you are referring to with reusing passwords is "credential stuffing".
Storing both on one device sounds like a pretty stupid thing to do. Sure, it's convenient, but you are putting a looooot of trust into the assumption that your single device will never be compromised.
Unless you mean the webserver they use being attacked, but that is just as vulnerable for 2fa. The only difference is the user cannot screw up and use the same credential on multiple websites with 2fa.
evilginx will happily steal most other 2FA (TOTP, ...).