We already know that thermodynamics and evolutionary pressures can allow for intelligent beings with a population average IQ of 100 (on the arbitrary scale we've devised for ourselves) to exist, since we humans do exist. I'm suggesting that a slight increase in that population average might be enough to make a large difference in how we collectively respond to mid- to long-term problems.

If you're arguing against that suggestion, it seems you would have to be arguing that human intelligence exists at a level that is nestled directly up against some kind of hard thermodynamic limit, even though a non-trivial portion of our species already exists on the other side of that line. I don't see how that argument could plausibly be made.