Arguably, there should have been an echo of him in the Nexus, just as there was of Guinan, and it would have worked better if the echo had then warped Picard (and Kirk's) experiences in the Nexus, but I do see that it might've came across as having to defeat Soran twice... But yeah, he should've been in there
Allow me to throw a wrench into your very logical response.
Kirk knew he was dead already once he realized he was in the Nexus and nothing mattered. While on the horses he indirectly tells Picard that he was only happy when he was a starship captain trying to make a difference.
So Kirk decides his last act is to die doing something he loved. In fact, his last words were "it was fun" so we know he died happy. I would say it's possible he died having never left the Nexus, as he couldn't be happy anymore in the quasi alive state the Nexus gave him.
I'll see your wrench and raise you a hammer.
Assuming that Kirk did in fact die in the Nexus, where does that leave Picard? Some kind of weird after-death dream of Kirk's?
And let's be honest, all of ST is just a grand story in Benny Russell's mind.