Replicating the case is easy, and the emulation is getting better every year. If the last A500 does pop its clogs then it's at least comforting to know that a fake emulation replica can be built that looks exactly the same.
As for last real A500...hmm... I'll guess thousands of years. There's stuff in the ground they keep digging up which are thousands of years old, so something that is not going to be weathered should last indefinitely.
Okay, capacitors would be dry - they would need replacing with a future equivalent, but they would be easy to fabricate. So I don't think you need worry about them dying. :-)