It looks like you could replace it with a name brand deraileur and a deraileur hanger. All the connection spots are available. And the quality would probably be a lot better.
Actually, a couple of places (including Gaerlan) told me that the 7-speed Neos derailleur is a proprietary part and cannot be swapped out.
I have heard anecdotally that getting spare parts from Dahon can be tough, although I've gotten warranty parts without any issues.
I do think you'd have issues getting a new part in 4 or 5 years. Used may not be a big issue. I expect that unless you whack it pretty hard, though, it should be usable -- or in the worst case scenario, rebuildable -- for a very long time.
Neos rear derrailleur bikes come with an additional derrrailleur hanger to mount any regular derrailleur if the customer wishes ( and pays ) for.
The Neos derrailleur is designed and built to work especially good with small wheels. The ground clearance is vastly improved over regular derrailleurs, the side clearance is on par with Shimano's newest XTR shifters, maybe better.
They shift crisply and much better than most " regular" derrailleurs out there.
So far I have not heard of anybody having problems with a Neos, nobody certainly asked me for a replacement derrailleur ( albeit I have some in stock ..... )
I agree with Thor, the Neos is just a cheap plastic covered stamped out steel derailleur, but it shifts crisply with the Sram twist shifter and the 11-30 freewheel. It's also tucked in for a smaller fold. It's a little harder to change the rear wheel, you have to pull the chain back by hand to pull out the wheel. Carry a rag with you!