If you're playing the game as if you are Kait, the decision to save Del fits her particular experience and story. He was there for her through all of gears 5. but as a player and viewer of the story, saving JD makes so much more of an interesting story and allows for way more dramatic beats.
Yes, JD was boring in gears 4. But in 5 they leaned into the legacy of marcus fenix, and what it's like to have the legend that saved the entire planet as your father. How do you live up to his accomplishments? You can't. JD failed to do so. he fucked up hard and is a bit of a piece of shit. His dad wasn't a great father to him, and meanwhile he was reminded all his life how his dad was this monolithic, legendary hero. That would fuck a lot of people up. The parallels between JD and Marcus are infinitely more interesting then the dude whos the sweetheart, techy side-kick friend. In addition to all of that drama; now that JD has lost Del, he finally has a real connection to his father for the first time in his life. A connection in which they both lost their best friend/brother. This is something that can finally allow their relationship to heal and move to a new place.
What story elements/dramatic moments does Del living bring that can rival all of that? He can go through the arc of losing a close friend, but he just doesnt have enough connections and emotional beats that tie into the other remaining characters to really make for impactful scenes imo.
