Oil viscosity is always a difficult compromise, even for automobile makers.
One should first understand that MPG is almost entirely determined by the HTHS viscosity and not really by the SAE viscosity. The same is true for engine wear, which is determined more by HTHS viscosity than the SAE viscosity. (Friction modifiers and antiwear additives are also crucial for MPG and engine wear.)
The change in fuel economy between HTHS viscosity 2.6 and 5.0 could be as much as 5% or more -- a significant impact on the wallet.
While it's true that higher HTHS tends to protect more, gasoline engines operate at lower torque and higher RPM, which decreases the critical value of HTHS for oil-film breakdown (which happens at low RPM, high torque, low HTHS viscosity, or a combination). So, most modern gasoline engines can tolerate HTHS viscosities as low as 2.6. One should also keep in mind that HTHS viscosity shears as well and a starting 2.6 could shear to as low as 2.0 after as little as a few hundred miles. Also, few people intend to keep their cars more than a few hundred thousand miles, and the lower-wear benefits of higher HTHS oils may not be needed for anyone who intends to sell their car after 200,000 miles or so.
All this said, I'm a high-HTHS guy, curently running 4.3.