The flip side of that is Joseph may not develop and if we pushed the boat out to keep him we'd regret not taking the 1m off Swansea when we had a chance. Him going now could be a blessing in disguise as we get 1m for the potential of player who may never actually go on to fulfil that potential.
The likes of Jerome Sinclair, James Wilson, Nile Ranger, Frederico Macheda, Adnan Januzaj etc were all considered more highly than Joseph at the same age and they flopped massively.
I'm not suggesting that will happen, but the game is littered with wonder kids who can't make the step up to elite level. Swansea will be thinking they got one over on us at the moment but in a few years time we might be thinking we mugged them off. You never know and only time will tell.