Just saw Blade Runner 2049, and the themes within the film are astounding! The crux of the film is that there was no real decoy, it was purely on paper. K/Joe is utterly, completely, unequivocally unremarkable. Thus his name is Joe, like an average Joe.
Joi does not possess a soul because she is completely fake. She is the other side of the replicant coin and is made solely to please and coddle her owner/lover. The giant pink Joi on the bridge calls K by his name "Joe" as well, and her entire branding scheme is that she will be "anything you want." K wanted to feel special, so his Joi always reinforced this to him, but it was never real. Joi is K's fleeting dream of being special, and once her emanator is destroyed, K learns he is not special.
Wallace posed a question about whether Deckard was moved by love or programming, therefore there's no doubt whatsoever Deckard is fully human. The original movie is about a bad man finding his humanity through the grace of a machine. Wallace's question is not "are you human or robot?" but pondering what the difference is, if love is just a chemical, and if we are products of biological programming or something higher like a soul. The ultimate takeaway is that it does not matter. In fact, the only thing that matters is what we choose to do with our lives.
In summary, 2049 is about dreams and delusions. K wants desperately to be special, Joi tells this to him constantly, and he instantly assumes all the evidence points to him because it's his dream. He becomes deluded and forces himself into the situation, even if it destroys him. He thinks this is what it means to be human, to grapple with one's humanity. He is torn between two sides telling him what his identity is and should be, either by the LAPD who informs his identity as that of a slave, or the resistance which informs his identity as that of a free replicant.
When his delusion is shattered by meeting the pink Joi, he chooses to follow his own path and not let anyone tell him who he is or what he should do. He makes the most human decision and takes his life into his own hands. He saves Deckard for the same reason Roy did at the end of the first film. He wanted someone to remember him, for his final decision that fully validates him as human to not be in vain. No one else gave him his identity, only he did, and his sacrifice ensured forever that he was by every metric a human being, even if the world would ultimately forget him.
The ending of 2049 is something that should be cherished by all who viewed the film, especially in our own real-world contexts with respect to a father figure. Whether you don't have a father (or never knew your father), or whether you do have a father (and know your father), the takeaway message is Deckard's own words to K: "to love someone you have to be a stranger."
K's memories (his "best ones") were Deckard's daughter's. In this sense, the truest part of K knows Deckard as a father. When asked by Deckard why he's gone to all this trouble to help him, K remains silent, a stranger in the face of a man that's as real a father to him as memories could suggest, and allowing Deckard the reunion he deserves. It's that human moment that ties up K's evolution as a character; a character lost in search of meaning and something real discovers his humanity and commits a selfless, loving act before he dies, gracefully, in the snow. A most human moment amid the film's cool, detached alienation. It's poignant and understated and I suspect many people miss that nuance in how it relates to the character.