To me it's just even more so about "you can only control the things you can control". Brady fought to a reasonable length. When it became a prolonged long shot that was going to be more of an investment than the reward was worth, he stopped. Kraft from the beginning was faced with options that only were bad one's that had very little if any chance of succeeding.
It's valiant to fight for what you believe in. It's stupid to fight a clear losing battle just for show in the face of more important things, like running and leading a team that's trying to win Super Bowls. Inevitably 4 games wasn't worth spending more of Brady's life in court and potentially being tied up in a Supreme Court case. Just like a 1st and 2nd round draft pick wasn't worth Kraft turning himself into Al Davis (Davis was fighting for a lot more than draft picks). There's a sense of proportions and a sense of what is realistic that you have to keep in mind in these situations. You never do things just for show. An appeal by Kraft would have been symbolic but ultimately meaningless. A suit by Kraft would have been futile and ultimately damaged his reputation with his working partners beyond repair. And at the end of the day, even if you are livid at Kraft for not going full bore, he's going down as one of the greatest owners ever, in the HOF, and the guy who is getting a big chunk of the credit for turning the franchise around.