A seagoing vessel is probably a better analogy (boats fall apart just floating around) - while there are ancient boats still floating, pretty much everything in them has been replaced at one point or another. In a dock - which is a pretty benign environment. It may be impossible to disassemble the ISS and replace the bad parts in orbit. Look at how much effort goes into just putting a new refrigerator on the outside of the thing. We just don't have the technology to uncouple a module, re weld a section and bolt / weld it back together again.
We likely should be working on that ability but this sort of construction and repair isn't sexy enough, I guess. Space is hard. Very hard. Budget constraints have made it so the US and Russia are really just jogging in place on the ISS - very little real research (such as in orbit assembly) is even being contemplated.
ISS-2 will be put together on the ground and boosted into segments and snapped together in orbit, but it won't be 'built' in orbit. There won't be any machining, welding, wiring, insulating or any of those types of functions done in space. We are no where near being able to do that past the sound stage.