What is your opinion on "why outdoor lighting -LED or any- is sustainable in design?"
I think an argument can be made? Sustainable should meet three criteria(Social, Environmental, Economic): Bearable means it meet Social and Environmental; Viable means it meets Environmental and Economic; Equitable mean it meets Social and Economic. In other words Sustainable is socially accepted, has no negative effect on the environment and does not have a negative effect on our economy, also easy to maintain.
Some believe that LEDs is the answer, and that it's the only product that can be considered Environmental and Economic. The color of the light, the type of illumination can be very different from brand to brand, this is an ongoing "standard" looking for the best solution yet.
All lighting fixtures should be sustainable in some way or they are a fraud. The ideal criteria for any fixture to contribute the least to pollution at the end of the life should be no lead, amalgam, phosphorus, sodium, plastic, so even in the LED world, arsenic is used in production.