While this would be the case, if someone is going to the lengths creating a save editor then they likely could just make it unlock everything and then have no need for gold anyway?
Not really, Your purchases and treasures are stored on the server, so every time you reconnect you would lose what you edited. Right now you could hack your game to have 1 BILLION gold, but as soon as you log in it will revert back.
As for the content, I don't know if it is already in the app or if it is an extra download, or needs a server verification to unlock.. If it is already in the app and doesn't need verification, then yes, you could in theory unlock it and play it without ever going online.
Point being though once everything is unlocked is you never need to go online again (aside from treasures, but then paying players don't get those for playing offline either), and playing online would actually make it worse for that individual, because it would lock out the game. So for most not going online (doesn't change the game-play) and not having treasures for a completely free game is a reasonable trade off.
Not one I'd agree with (if a company does a good job they deserve their money IMO), but lets face it there are a lot of clever hackers out there.