I'm no evolutionary biologist, but it seems like the increased intelligence (or more accurately, cognitive performance) may not have had all that practical a benefit until just recently in our history anyway.
I accept that the verdict is still out on tDCS, but am clamoring to reach that verdict.
This is a fairly cynical point of view. But safe. We are not talking about medicine, we are talking about messing with otherwise perfectly healthy people.
Off the top of my head, vitamins and vaccines are the only side-effect free wonder drugs for healthy people. Similarly, clothing is the only body augment to fit this category. (And even then we get it wrong sometimes. Lots of examples from women's fashion in the past and currently the debate about shoes's effects on the knees/back.)
Vaccines have well-characterized death rates and is why things like the smallpox vaccine stopped being administered as soon as possible even though it'd be comforting to have built-in immunity against that nasty biowarfare possibility; and some vitamins have been shown to increase mortality in RCTs.
That's obviously wrong, because the brain is so metabolically expensive that if it was not pulling its weight, it would long ago have shrunk considerably. More intelligence is always better - if the intelligence is not too expensive. (I cover this in the link.)