A less diversified portfolio is a riskier portfolio. In the short term a riskier portfolio can even outperform a less risky one, but over the long run that is less likely due to the impact of longer drawdown periods for the riskier portfolio.

That being said, I think that return alone is not a sufficient measure of the quality of our investments - I am not sure I will be very happy if my money finances companies that make life on Earth less desirable, irrespective of their returns.