A great fighter once said: it ain't about how hard you hit, it's about how hard you can get hit and keep moving forward.
Mary Ann Creed tracked down Adonis and adopted him as her own kid. And it's clear throughout the film that she loves Adonis and he loves her.
When she gives him a pair of boxing shorts styled after Apollo's iconic American flag ones right before the climactic bout, the card attached to the gift wrapping is signed "Ma". It doesn't matter if he was a sign of Apollo's infidelity — Adonis and Mary Ann were as good as mother and son.
And those shorts carry both of his names, Creed and Johnson. Neither Mary Ann nor Rocky are asking Adonis to cast aside his birth mother or his own history to become his father, they're honoring the person he himself has become.
Very shortly after meeting Rocky, Adonis starts calling him "Unc'." Rocky doesn't object.
Adonis' reaction to learning that Rocky has been diagnosed with cancer. The very fact that he is so concerned for his mentor's health is moving in and of itself. Furthermore, when he finally confronts Rocky about it in a reasonable manner, Adonis makes it clear that he will not participate in his upcoming bout unless his trainer accepts the necessary medical treatment:
Adonis Johnson: If I fight, you fight.
Adonis taking care of Rocky and training for the big fight.
Rocky doesn't let his chemotherapy slow Donnie down either, forcing the kid to train even when they are at the hospital, much to the amusement of his nurses.
During the big fight, when Adonis is nearly knocked out; he briefly sees flashes from different moments of his life - the last image? The one that brings him right to his feet? The memory of Apollo, in his prime, grinning in the middle of a fight with Rocky. As if his old man personally told him to get off his ass and see this thing through.
Rocky: Look at me. I never got a chance to thank Apollo for helping me out after Mickey died. But it's nothing compared to what you've done. You taught me how to fight again. And I'm gonna go home and I'm gonna fight this thing. But if I fight, I want you to fight, too. I want you to go across this ring and knock that son of a bitch down. Can you do it? Say it.
Conlan personally congratulates and thanks Adonis after the fight, and the amount of respect he has for his final opponent is obvious.
Conlan: You go and get it, chum. You the future of this division. You wear that name with pride.
After the last fight, Adonis is asked what he would say to his father if he were here tonight. The look on Donnie's face makes it clear that he never thought of that question until now, and the first thing he says is that he would tell his father that he loves him, and that he's proud to be a Creed.
At the very end of the movie, Rocky and Donny walk up those steps one last time. You know the ones.
The film's plot is heartwarming, as we see Rocky passing the torch.
Furthermore, the sheer number of subtle to obvious allusions to the past Rocky movies is both extraordinary and touching. Ryan Coogler and company really went all out to delight not only a new generation of fans, but also the people who grew up watching Rocky from the beginning.
Michael B. Jordan (who earlier the same year had suffered the indignity of Fantastic Four), saying that he'd absolutely be up for doing more Creed films, and he's just as excited as the fans to see where Adonis could go from here.
Both Coogler and Sylvester Stallone himself have said that the film is a personal dedication to Stallone's oldest son Sage, who died of a heart attack when the film was in pre-production. Also, the picture that Donny sees of Rocky's son midway through the film is a Real Life picture of Sage, wearing boxing gloves and posing with Stallone in front of a punching bag. Coogler insisted on using the picture since he thought it wouldnt be right not to have a representation of Sage in the movie.
In the wake of Stallone controversially receiving the film's only acting nomination (in the second year in a row where every single one of them was white), he offered Ryan Coogler to boycott the ceremony just like several black celebrities were planning, saying it was entirely thanks to Coogler that the nomination happened. Coogler in turn told him to go, and represent the film for them all.