Re: Should Illia and Decker have survived ?
ISTR wasn't there talk that he was Matt Decker's son? So while it's clear Kirk had some pull in getting Will Decker the captaincy, and Decker certainly resents it when Kirk shows up again, it's also possible that there might have been latent feelings because of Kirk's involvement in the incident where his dad was killed (even though obviously Kirk didn't kill Matt Decker, Matt Decker killed himself).
It's generally accepted (but not cannon) that Will Decker was Matt Decker's son, so it's possible Will's feelings about his father's death (not Kirk's fault) may have affected his attitude toward Kirk. OTOH, Decker had more than two years to confront Kirk and get his feelings off his chest. That he wouldn't do that but would passive-aggressively channel his frustration over his father's death toward Kirk to get back at him for taking the Enterprise
away is petty.
"He clapped his captain—his friend—on the shoulder. Yes, this man was very much like James Kirk, in all the ways that mattered." --Christopher L. Bennett-- Star Trek: Mere Anarachy, The Darkness Drops Again