Why can't he be worse? He can easily be worse. Different is not necessarily better. The guy's a career journeyman whose stats have declined steadily from their peak over the last several seasons. You may see some more completions downfield, but if they're offset by more picks, it's not going to matter. Sure, it gives the illusion of progress, but it's really just motion and not progress.
That said, we do have to look seriously at whether or not we think Cassel might be an actual upgrade. Just because I'm saying he's not necessarily an upgrade, it doesn't mean he's not, in fact, an upgrade.