We're living in interesting times for this "conventional" wisdom! One school of thought argues that while there might be a theoretical relationship between high pressure and low rolling resistance, but the real-world effect has been exaggerated by poor testing methodology, which usually involves a circular drum as a substitute for a flat road. And that the benefits of achieving low rolling resistance are offset by the losses due to increased vibration.
Of course, this school of thought is largely the product of one man, Jan Heine of Compass Bicycles, but it's interesting to read and consider. Personally, I made the jump from 23mm road tires at 120psi to 48mm slick tires at ~40psi this year, and I have to say, my average speed didn't take a hit at all on the road and am much happier and more comfortable.
https://janheine.wordpress.com/2018/05/02/testing-tires-isnt...
https://janheine.wordpress.com/2016/03/09/tire-pressure-take...