Getting rid of Sam Mitchell is no different to Geelong turfing out the likes of Johnson and Chapman, and unlike Geelong, Hawthorn will probably get something decent back for Mitchell and have also immediately filled his spot with another quality player 10 years his junior.
As far as replacements, you probably have a point with Chappy, I can't remember who replaced him - whether it factored into the game time of Guthrie, Caddy, Duncan etc.
With Johnson, I think the club were hedging their bets on Menzel and McCarthy being that 8-10 year junior replacement rather than needing to seek a trade from outside. Our forward line depth was such that he still could've fit in, likely at the expense of Bartel in hindsight. But that appears to have been the logic. Moving clubs brought the best out of him. I would've liked to keep him, but I'm glad he went on and did well.
Edit - as mentioned above, cap space to fit in guys like Danger and Hendo was the other factor. We'd just missed finals so moving on Stokes, Kelly and SJ for Hendo, Smith and Danger seemed a better bet to get us back there. Again with Chappy, probably more of a fail. Who'd we ship in - HMac and Clark? Ha, that went well.

