What if the drugs just make us... better? As in happier, less stressed, and more suited to what modern life requires from us.

Happiness isn't necessary to pass on your genes. Most animals, especially prey animals, spend much of their lives scared, because death is inevitable otherwise. We've now created a safe and pleasant environment our ancestors could only dream of, yet many of us are still unhappy. There's no real evidence we were broadly happier in the distant past either, we're just a different kind of unhappy now.

Seems to me we've developed a tool that makes people feel better and function better, perhaps by blunting some of the vestigial leftover emotions we needed when every tree might have had a sabre-tooth tiger hiding behind it.