Why is Emirates not considering buying or leasing used A380s?
(Source: IATA)
Used airplanes are costlier to operate and maintain. Take Ryanair for example, they have 420 Boeing 737's with an average young age of 6.8 years. That age is very important.
The graph above shows the direct maintenance cost of an A320 for the airframe, similar graphs are available for the other systems. Note when the first period ends.

First-Run is the initial operating years, often referred to as the honeymoon period and generally
considered the first 4-6 years of in-service operation. The structure, systems, and components
are new; and there is less non-routine maintenance and material scrap rate.

The following serves as both a proof and an answer: Airbus is buying back the older A380's in the new deal with Emirates for new A380's.

Airbus (AIR.PA) will have to buy back or find new homes for some of the older A380s currently operated by Dubai’s Emirates as it finalises a deal to sell new superjumbos to the Gulf carrier.
(...)
The decision by Airbus to buy back aircraft to facilitate the anticipated deal comes as the aviation market struggles to absorb some of the first A380s, which entered service a decade ago.

The Reuters article is from November 12, 2017. It's no wonder you see older Boeing 777's next to 727's:

Something Airbus is wary of, they don't want to see an A380 in the boneyard so soon.

Airbus is seen as keen to support the second-hand market to avoid having to break up one of the planes, a potential public relations setback as it tries to breathe new life into the iconic European programme following recent production cuts.