Sigh. Alright, Jax haters, I'll take the bait and try to put a few things into perspective for you:
1. Jax serves as an added layer for Jacqui's growth. She is very proud of her father's accomplishments in the SF and thus, wants to follow in his footsteps. Furthermore, she wants to get revenge for what happened to him, mainly because of Ermac disfiguring him and Quan Chi controlling him. Jacqui speaking to Jax via device while he's in the Netherrealm shows that she still cares deeply for her father, despite the fact that he is a former revenant. This is further examined when Kung Jin makes a snarky comment about her father and she tells him off. If for no other reason, Jax is there to show you the similarities and differences between the two of them so fans can truly see that she is his daughter and will be the way he's survived in future games IF they choose not to bring him back in the following titles.
*Side note: After being defeated by Johnny, Revenant Jax harms the psyche of Sonya, which is one of the only times we really see a chink in Sonya's armor, as she seems really distraught by what has happened to her best friend. After Johnny convinces her it isn't him, she realizes what must be done in order to free him and the others and is able to get her head back in the game.
2. The focus of the game is the new team of heroes, who are SF. Jax, being the former CO, fits in perfectly. It really is a passing of the torch type of thing, though, because of the fact that the old guard is present but they mostly serve as filler to Cassie, Jacqui, Takeda, and Jin. For hardcore fans of the series, NRS had to give the originals their clout. This is the reason that Jax, who debuted in MKII, was not written off as being completely defeated after the events of the previous game. They had him come back and get revenge on the one who ruined his life, Quan Chi, by beating his ass and securing his arrest. This helps give Jax a sense of closure and shows no matter how much time he takes off, he's still a credible fighter. I'm sorry but no, Kenshi could not have "easily pulled off that chapter." Lastly, how silly would it be for Jacqui to be the only newcomer who's parent isn't in the story mode?
3. On a smaller note, Jax being in the game is connected with the Predator DLC. As I'm sure you know, he will be playing the part of Carl Weathers. NRS likely saw this as a way to really cash in on the guest character even more by giving players the option to play out the epic battle their own way.
Any single way you slice it, this game benefited from having Jax in it. Not sure how you figure having characters like him in it did the new characters a disservice, as each of them were given proper screen time, with an exception of Ferra/Torr. We see Kotal Kahn as a proud warrior, invested in protecting Outworld by any means necessary, complete with a flashback scene. We see D'Vorah as a cunning, traitorous opportunist and uncover her sadistic personality as she delights in the pain of others. Hell, we even learn that Erron Black is an Earthrealmer and completely loyal to Kotal Kahn because of the almighty dollar, not because he is a fan of democratic politics or particularly fond of him. What more were you looking for? The only thing they really didn't touch on that left something to be desired is the origin of Ferra/Torr.