Fox was never as good as most fans assumed. This isn't new for me. I thought then and believe more now that Hali is a going to be a better player. See here:
https://community.kingsfans.com/threads/trade-suggestion-fox-for-simmons-28.79959/
As for his regression, he put on pounds and lost his burst. I applaud him for putting in the work, but his trainer/agent and him made the wrong calculation. They thought the muscle would turn him into Russ 2.0, who is straight line like Fox, but is levels more beast mode. With the added muscle, Fox needs to zig and zag like Donovan Mitchell. Fox has never zigged or zagged. Don't think he'll ever truly be great at it. HIs ceiling is likely a longer Tony Parker. If he misses that ceiling, he's Leandro Barbosa 2.0. Average to poor shooter. Speed. Straight line. 10 year career.