Everyone always says that ChatGPT is bad now, but aside from complaining, what mistakes were made to cause it to regress?

Everyone always complains about functionality (rightfully so) and reminisces on 4o. I am in full agreement. But instead of just yelling at a brick wall, I'm curious as to what mistakes the developers made to cause such a regression? Was it their fault? Was it just inevitable?

We all know it sucks now, but WHY?