I suspect that Welling wants to avoid the "typecast curse" that comes with suiting up as Superman.
Also we don't know the full state of Superman TV rights, does DC/Time Warner have a hammerlock on them now, or does the fight continue?
Geoff Johns says No Superman, and that is good enough for me.
The only thing that will change John's and Welling's minds is a good story and a truckload of cash.
Think that the final call will be made by Warner/DC/CW, and that Tom will indeed be asked to don the tights one final time, as DC wants to have the JLA for TV before the movie version takes flight!