Sorry, I'm not seeing where casters are being nerfed. One spell (spiritual hammer) was buffed to scale better, but an exploit was fixed that doesn't allow it to be stacked with concentration spells; most people already felt that this was a problem with it. Banishment definitely got a nerf, but again it was seen as very broken and a no-brainer choice. On the other hand, other spells are getting buffed to make them more advantageous. It's way too soon to claim that spell casters are getting nerfed overall; from what little we've seen it feels like more of a wash as WotC attempts to make spells more balanced, just as they are doing with feats.
It's possible that better balance will feel like a nerf, but then it will affect all classes because it is also happening to things like Great Weapon Master and Sharpshooter. On the other hand, there are folks claiming that OneD&D is causing power creep. I think it's too early to tell, but none of the changes seem earthshaking to me thus far.