One of the characteristics of elegance that Matthew May talks about in his book "In Pursuit of Elegance" is sustainability.
On page 143 of "In Pursuit of Elegance", May defines sustainability:
Sustainability can be defined broadly as the ability to maintain something at a certain level, indefinitely.
Then he goes onto discuss two important implications of the definition:
1) To be sustainable, any given asset, no matter what is is, must be kept whole, without making significant trade-offs that undermine the capital used to generate and maintain it.
2) Sustainability hinges on the ability ot see finite resources as the very source of innovation.
Of the four characteristics of elegance that May talks about, including symmetry, seduction, subtraction, and sustainability, I find sustainability the most difficult to succintly define.
My impression is that sustainability means making things that 1) last a long time, and 2) are useful while they last, even in a secondary sense. I can understand that it is something that makes en elegant design or solution better and is more useful. I think sustainability butts against the idea of elegance and timeliness that I am also researching. While elegant designs are rarely timeless, they can be useful for a long time.
For example, when I upgrade to a new iPhone, my old iPhone can be used by my kids to play movies and games and listen to music and take pictures. It could also be used as a phone and communications device, but its performance degrades gracefully. If all of a sudden, the iPhone was not software upgradeable or applications stopped working with it, it would be less sustainable. Or say there is a laptop backpack that can have its laptop carrier switched out for a different size laptop carrier. That way, the bag can continue to function while what it is carrying changes down the line.
An annoying example of this is that most car's bluetooth systems become hopelessly out of date after a year or two, and are never upgradeable. So maybe my 2011 car can work with Siri fine, but my 2008 car cannot, and never will without a clunky third-party add-on. And no one wants to upgrade a car to get better bluetooth connectivity.
For my model of elegance, I think that it will be intresting to compare and contrast timelessness with sustainability. Maybe the two meet in the middle somewhere, or maybe they are each relegated to their own forms of changing functionality and usability.
Regardless, sustainability has become a much bigger issue lately. It's something that probably differentiates the 707 from the 787 (fuel efficiency), and differentiates the late-2012 minimilist low-powerered destop iMac from the 2003-era PowerMac G5 desktop (power consumption). But I am still not sure how to best incorporate it.
And check out Matthew May's book. It's a good introduction to the idea of elegance.