why the token is becoming obsolete?

Dropbox is no longer offering the option for creating new long-lived access tokens. Dropbox is now issuing short-lived access tokens (and optional refresh tokens) instead of long-lived access tokens. You can find more information on this migration here.

Apps can still get long-term access by requesting "offline" access though, in which case the app receives a "refresh token" that can be used to retrieve new short-lived access tokens as needed, without further manual user intervention. Refresh tokens do not expire automatically and can be used repeatedly. You can find more information in the OAuth Guide and authorization documentation. There's a basic outline of processing this flow in this blog post which may serve as a useful example.

Thanks, and how long is a regular short-term token valid?

Dropbox API short-lived access tokens are valid for several hours. You can have your app get the actual expiration information from the 'expires_in' value returned with the short-lived access token from /oauth2/token.