Sure if one is minimalism (using a few services) Once a storage needed (load of brain, password manager, paper etc.), the "just" goes away.
> without any entanglent to some app, device
Again If said storage needed, this is not advantage anymore.
Using a security token is more like a password manager with random passwords everywhere than that (the attacker needs to get access to your password manager to get access to your accounts; it's not enough for someone to hack a single site you use), but more secure because it's generally not copyable and the attacker needs physical access to use it. (A virus on your computer can't clone your security token, even if it's plugged in.)
Does there exist FIDO key (other than phones) that require a password to "enable"? For example, when it's plugged into a new device, the key locks until you input some master password?
2FA is still an option (e.g. 1 thing that you have + 1 thing that you know), with the hardware token representing a more secure alternative to phone SMS messages.
> Isn't this almost close to using the same password on every site?
To get the a shared password, you have to hack one of hundreds of different services that password is used on, or phish the user, and penetration often goes undetected for years.
To get the physical secret, you have to rob the user, who will notice they've been robbed the next time they attempt to login. Additionally, trying to login to a phishing website won't automatically auth them with the real website - I can't do secure key exchange algorithms in my head, but a hardware device can.
For the time being, passphrases + TOTP 2F are sufficient, but I will look forward to being able to use a security key instead.
Yes, IIRC it's impossible (or nearly so) to phish U2F.