Climate change will do this. No additional conspiracy required.
There's research showing people get incrementally less intelligent as oxygen ratios get worse. There's also research showing that plants (which really all of our food depends on one way or another) become less nutritious and more sugary/starchy as carbon dioxide ratios rise. That's before we even factor in things like endocrine disruption from plastic particulate ubiquity and dozens of other pollutant effects.
We really are the frog in the slowly boiling pot, and even when citing sources on this kind of thing people would rather argue about it. 🤷♂️ (I'm not going to bother. Can be looked up easily enough if you're so inclined.)