API tokens are a very small narrow part of the authorization universe. Having a shared secret relies on a trust relationship between the resource server and the identity provider that does not exist between, say, my SaaS backend and Google or Meta's login system.
The OP was talking about sessions (which include session cookies and API tokens). I'd argue these use cases are far more common for the average programmer than tokens and signatures that are used for federation, but I'll bite the bullet here:
JWT is a serviceable solution for service trust and federation. This use case often just requires a very-short-term token, so lack of revocation support is not an issue. Replay attacks are still an issue, but they can also be prevented with single-use nonces that are included in the token claims.
The OP's take (and my take as well) is that JWT is rarely the BEST solution for this use case. You kinda have to use it if you need to implement a standard that mandates JWT such as OpenID Connect. But OpenID Connect is a great example for a place where JWT was used, but was never really necessary. If you do use the authorization code flow securely (on the server side, with a strong client secret and proper CSRF protection) you don't need the ID token. In fact, you don't need to use any cryptography at all! Just like random session IDs, you've got a stateful solution that works reliably without any cryptography.
If you cannot do a series of authenticated network requests between HTTPS endpoints to verify trust, then a signed payload could be useful, but you've got better standards than JWS/JWT for that. That's all.
> If you do use the authorization code flow securely (on the server side, with a strong client secret and proper CSRF protection)
This restriction precludes all desktop clients, mobile clients, and webapp clients -- any place where you can't trust the client code to protect a secret.
I don't exactly disagree with you: Security becomes much easier once you rule out handling all the hard edge cases.
PKCE, OAuth 2.0 for Native Apps and the Device Code flow are a thing. In practice all of these clients work so well with OAuth 2.0, that the implicit and resource owner password credential grants have been removed from OAuth 2.1 and are the latest OAuth 2.0 BCP forbids the password grant and strongly recommends against the implicit grant.
For what? For just authenticating a client application to a server without getting access to any user data? By all means. You usually don't need the client credentials in OAuth 2.0 if you don't want to use it.
But if you want access to user information, you can't just give access to an app that has a shared secret key to a all user data for every single users in the planet. You need to authenticate the user somehow, and sharing the user's password directly with the app is not a good idea for many reasons.
Even if you trust the app to be "written without any bugs", the moment you allow the password grant in OAuth, your service cannot use CAPTCHA, MFA or Passkeys efficiently. The password grant does not allow any of these, so this app that you've let through becomes the weakest link for all cybercrime groups that want to attack your service.
