Here’s my theory: with his ungainly size, T-Rex might have had a tough time hunting compared with smaller, faster, more lithe predators like Velociraptors, who likely traveled in packs to better flush out and entrap their prey. So T-Rex may have become more scavenger than predator, waiting for the smaller predators to bring down their kill before ambushing them and sending them scrambling to get away from those huge jaws. Then T-Rex would peacefully enjoy munching on the newly killed carcass. So his arms become vestigial simply because, as a scavenger and a thief, he no longer needed them to snare his prey, which was already dead. What do you think?