Brook prevailed in the British grudge match, stopping Khan in the sixth round to win bragging rights.
In a rivalry that had been brewing for 17 years, Brook dominated proceedings and had plenty of offers on the table.
However, the former welterweight world ruler snubbed both offers and announced his retirement last month before Khan followed just weeks later.
"I must say I'm really happy to hear the retirements of both Kell Brook and Amir Khan," Johnny Nelson told Planet Sport.
"One of the most frustrating things about our game is seeing good fighters carry on when they should've given up years ago. But the youngsters look at those good fighters you know and think 'god this guy kid is rubbish'.
"When these guys are actually top draw. So, Amir Khan and Kell Brook have actually walked away where they got everything still intact and made a lot of money.
"I know Amir's prides hurting but he can put it down to the fact that he just didn't have it in the ring. That's what he'll put it down too but I'm just glad they've both retired.
"He'll sleep well," said Nelson when asked about how Brook will look back at his career. "You get fighters that go on too far and they have bitter memories of their relationship with boxing, which is sad.
"And they have bitter memories because they've gone on too far and people forget how good they were. But Kell had done the right thing, he's boxed on, he's stopped when it was time to stop, and so has Amir Khan."
Best Khan performance?
Amir Khan leaves the sport having won two world titles, a silver medal at the 2004 Olympics and an incredible resume that includes Marcus Maidana and Canelo Alvarez.
Johnny Nelson claims Khan is the fastest fighter he has seen live and picked out the best performance he witnessed live by the Brit.
"The best I saw Amir Khan was when he boxed Devon Alexander. I've always rated Amir Khan's speed but when you see it live, this boy is fast, blistering, on point. And to me that in my opinion was his best performance.
"It's okay to have hindsight and look back and say I wish I would've done this, or I wish I would've done that, you didn't because you didn't want too. Our sport is littered with fighter that wished they changed something when it was too late at the time that's what they really wanted to do.
"The money that was made after those wish list were made. I think for Amir, he threw one more dice, he gambled it one more time and he lost. So did Kell, he made the same gamble but he won. So I'm glad Amir has retired."