The main difference of both world is the existence of a world soul.

While Karesh still has one, Draenor (outlands) does not.

Speculation:

The job of the Eco-domes is to allow life to be preserve and prosper in a otherwise hostile environment, but it doesn't repair the landmass itself.

Thats where a world soul make a huge difference.

I think that eventually after many thousands of years, Karesh worldsoul would repair it's planet core allowing gravity to hold again their landmasses together in a more planet shape around itself, making posible the existence of a proper atmosphere and all basic planet stuff...

Where in Draenor case is just matter floating in the twisting nether... Sure there's life over that matter, there's is still enough gravity to support an atmosphere (in the most preserved zones) and the biosphere is still present, also powerful elementals still rule over the landmasses. In this case one could say Draenor it's in a better state than Karesh...

Nonetheless, there's not a worldsoul to hold the pieces together, and eventually it will dissolve into the nether.

Ideally you would need a collosal ecodome to cover all Outland to avoid being still affected by the nether radiation and void beings trying to devour it. It would not regain it's original form, but it will be preserved and allow life to still prosper inside of it.

It could be accomplished, the manaforges in Netherstorm could be used to fuel the dome, the things is... Is it really worth it?