I mean I was actually fine with him going robot arm-free for a while, or forever. It would make sense as a character growth thing. That said in a world where you CAN replace your arm, I don't know why you WOULDN'T unless it was for some moral reason.
It would've been super easy for them to write in a bit of dialogue at the end with PB saying "It's a shame about your arm. Don't worry, I'll make your next one even better" and he just says "Naw, I think I'm gonna live without it for a while." Like it's weird that they didn't have a line of dialogue acknowledging it. I guess you could call it subtle writing but it seems odd no one's calling attention to it.