That's a very romantic view of the world, and as a huge Brady fan going back to his Michigan days, I really hope you are right.
However, I doubt it will happen that way.
The majority of recent good QBs don't end up walking away from the game and their team on their own terms. Elway is the obvious example, and Marino and Kelly walked away as well.
There are guys who stayed with one team but had to walk away early because of injuries. Aikman is an example of that.
But it's hard for a star QB, the ultimate competitor, to just flick the switch off. Manning's injuries played a part in the Colts parting with him, but he wasn't ready to leave the game. Same with Montana. Ditto Favre, who did the ultimate **** YOU to Packers fans by eventually joining the Vikings after a year-long stint with the Jets. Warner bounced around a bit after the Rams. Cunningham, Bledsoe, McNabb, McNair, so many others couldn't just walk away from the game when their games started to decline.
And Brady is more competitive than all of those guys. He's spent his entire life working to extend his career. Every meal, every workout, every nap is designed to give him an extra year or two or three. You think he's walking away from the game?
That's not how he's wired. He was the 6th-string guy at Michigan, the 4th-string guy in New England, he pushed himself to primary backup to quality game manager to Pro Bowler to HOFer to GOAT. You really think he's going to just turn off that competitive drive when his game slips a bit?
His contracts are structured to make it as painful as possible to cut him, while being as team-friendly as possible. Even if his game slips a bit, he's making only a touch more than Ryan Tannehill or Colin Kaepernick so he doesn't need to justify a top-3 salary anymore.
A lot could happen between now and Brady's retirement. Injuries may rob us of seeing the story play out. Who knows? And maybe he could just one day wake up and decide he's done with it. But knowing everything we know about Brady, I just can't see him spending so much of his life trying to maximize and extend his career, then walking away earlier than he has to.