Most English speakers are bilingual (because most English speakers are not native speakers) and the cognitive benefits of bilinguality are the same regardless of which languages you speak. I don't see how there's anything inherent in English as a language that makes people stupider.

I don't have any data on this but I would argue Zoomers and Generation Alpha probably speak better English than their native language and even if they speak their native languages well, it's a completely americanized version of their mother tongue. I would argue not being able to speak your native language properly and only communicating via Discord/Youtube/Reddit-influenced American bastard language is a form of stupidity.