Adlerdriver

Mark, this statement above is not correct. Pressurization of an aircraft is the equivalent of inflating a balloon. The pneumatic system of an airliner pumps a high volume of air into the aircraft interior. This in-flow of air is regulated by an outflow valve that continuously allows a certain amount of the air in the fuselage to vent overboard as "new" air is constantly introduced to the cabin environment. So, at any time during high altitude cruise, every molecule of air in the "inflated" fuselage is straining against the interior of the aircraft - pushing outward. The lower (closer to sea level pressure) you make the cabin altitude, the more air molecules are present within the fuselage and the higher the interior pressure on it. "Inflating" the fuselage to a higher pressure and then "deflating" it as the aircraft lands is what causes the stress and fatigue on the airframe.

It's a combination of both. In older (non 787) aircraft, in order for an aluminum fuselage to be structurally capable of withstanding the additional stress of being pressurized to lower cabin altitude, it would need to have all aspects of its structure strengthened. This would obviously mean more material during construction (cost) and more weight during the operation of the aircraft. More weight within the aircraft structure means less available for fuel, passengers and cargo (less profit). The other factor involved is the source of the air being used to pressurize the aircraft. Engine bleed air is typically used to operate the pneumatic systems involved with pressurizing the aircraft. So, using more of this bleed air from the engines to increase the pressurization within the fuselage means the engines will operate less efficiently and use more fuel.

The 787 carbon fiber fuselage allows Boeing to pressurize the aircraft to a lower cabin altitude than a similar aluminum design. Since that aircraft also uses an electrical pneumatic system rather than the traditional bleed air system on most aircraft, they can accomplish this increased pressurization with no additional demands on the engines. Nothing come for free, so I would imagine the electrical demands are considerably higher. This may demand more from engine driven electrical components but obviously that was deemed to be an acceptable tradeoff by the designers. From what I understand, in actual practice, this electric-centered aircraft design has had some growing pains in terms of system reliability.

Cabin pressurization is a fairly simple concept and the physics involved with actually accomplishing it aren't going to change. You have to pump enough air into the fuselage to sustain life inside a tube that spends hours above 30,000 feet. It seems like Boeing was able to make some minor "breakthroughs" in fuselage structure and system architecture that allow them to get cabin altitude lower without major sacrifices in efficiency. They were able to drop it from typical ranges of 6000-7000 feet down to 5000-6000.

Getting it to sea level is probably not a worthwhile trade off for the extra cost involved. It's definitely not optimal when it comes to efficiency and aircraft operation.