The intention of that letter was never to get the players back to the table. Why lecture to the players then about everything the OWNERs were giving up? If he was sincere that is not the letter he should have sent.
The battle is going to court, everyone knows this. No letter by Goodell can change that. And as much wishing and thinking that Goodell has the best interest in the sport he simply doesn't. He is the watchdog for the OWNERs.