Forget the stats, there's only one area where Smith = Cassel matters.
Neither are good enough QB's to win a championship with unless you build a 1985 Bears/2000 Ravens defense and everything else falls perfectly.
Every year he's the starter is a waste, except for the folks that are content with 8 wins every year. Playing Smith isn't building towards anything, it's spinning your wheels.
Or as someone posted on Twitter, trading for Alex Smith is like stopping at $8,000 on Who Wants To Be A Millionaire - the money is nice, but it's not life-changing.