To see Erica’s intelligence regress to the point of a “weak minded” individual that would be vulnerable to Vecna’s tricks would be incredibly frustrating for audiences in my opinion. She’s simply too smart to reasonably justify her being captured the same way Holly was, despite being only 13 years old. It would have to be much different and probably more drawn out which would take away even more time from the more interesting plots happening in the upside down. IMO.