A starting job is little improvement if you're still constantly being punished for your performance and playing your game. At its core, I think Pop really believed he could take the "hard dog" approach with Lonnie, as he'd done with Tony or Manu or whomever, when Lonnie just doesn't have that personality at all. Instead of getting hungry and driven from getting benched and screamed at after his first defensive miscue, he just gets in his own head, and once his confidence takes a hit, he disappears into the background. That kind of personality requires a different, positive-reinforcement based approach, which I simply haven't seen Pop try at all. At least not on the court.
At the bottomline though, I don't at all disagree that a big part of the problem is on Lonnie. I'm still (maybe one of the only posters here) holding out hope for him, but it's no secret that he should be better right now. I definitely blame him for his passiveness, his non-improvement on the defensive end and high-reads in general, and overall not "stepping up". BUT, I think you can also perfectly point out his developmental curve (which again, was on a rise, with his bubble performance being a definitive peak where he DID NOT PLAY LIKE HE DOES THIS SEASON, only to come back down this season) and wonder whether the developmental staff (Pop mainly) it up.
I don't see how both things can't be true at the same time. Here in ST, people are always so absolute about everything - either he's trash and we gotta get rid of him, or he's just a season away from supers om. Either he was always a "beta male destined to fail" or Pop pissed on his cereal every day to get him like this. Why can't a middle ground be found? I see both things, and we all saw both things, happening these past years. Or were we collectively hallucinating Lonnie getting benched for Forbes and Beli, who proceeded to have league-worst defensive performances without a hit taken to their minutes and team roles? Nah.
Ultimately, you're right, the offensive side is all in Lonnie's head right now. I'm beginning to wonder whether he'll ever "get it out". Ironically, I agree with you also - it's not all Pop's fault. My original point is against the posters saying none of it is on Pop, or that Lonnie has not improved since his rookie season (flat out false), or that he was destined to fail from the jump due to some rando scouting report. That's where I draw my line.