What concerns, besides orbit decay and continued cost of operation (and potentially, a critical failure) are the motivations behind decommissioning the ISS by 2020? I mean, I'm fairly sure it would be cheaper to develop a weak-thrust motor that would move the station to a higher orbit, instead of just dumping it and building a new one, and it's probably quite some time before it begins failing as badly as MIR would while still operating.
There are some seals, on some of the modules that have a limited lifespan. The lifespans are in the 20 year range, but they are very hard to replace.
Some of them are on the Russian end of the station, (Oldest part of course, in terms of launch, but since also was built even earlier than launch for Mir II usage) and these are the ones exposed to the fuel/oxidizer for station reboost. Thus they are exposed to Hydrazine and MMH which is horribly corrosive, and even seals safe with those things, have a limited lifespan. I imagine these seals are buried fairly deep inside the module, and not greatly accessible for replacement.
Conversely, the Russians have talked about taking their modules and going home, if the US decides to end the ISS 'early' from their perspective which argues against my point.
The US side is much harder to support without Zvezda/Zarya for orbital control. US side does much of the power, and heat radiation, but not much of the orbital maintenance.
The orbital decay is not the reason for abandoning ISS at all. The orbit is constantly being corrected by the cargo ships, plus several modules have their own thrusters for orbit correction.
The last Mir expeditions tried to find and fix up small holes in the hull with an epoxide based glue. Not much luck. They found one, but could not find the others.
Salyut-7 station was left without a crew for several months. A power failure caused a loss of contact, so the ground control could not control its flight. And what's even worse, it had stopped the life support systems. The water froze. The pipelines were seriously damaged.
The failure to account for increased solar activity invalidated previous calculations and brought down Skylab from the parking orbit much earlier than anticipated.
That is, critical events are happening all the time. The plastics degrade, the metals corrode, micrometeorites cause damage from outside. And if left without attention a space station produced with the current level of technologies will quickly become a useless and dangerous piece of space garbage.
Another reason for deorbiting Mir was a need to spend resources on the newer ISS project. The Russians have had previous experience of using two stations at the same time. That was the brand new Mir and the old good Salyut-7. (They even flew in Soyuz there and back between the stations). Two stations are just too expensive to support together. So if the Russians or Americans or all together decide to spend money on something newer and better, the station will have to be abandoned.
I will use an analogy. As your car gets older it wears out. It gets more and more expensive to maintain as parts fail. At some point it is cheaper to get rid of it and buy a new car. This point may depend on how safe you feel in the car.
An old ramshackle rustbucket may be drivable slowly, despite having holes in the floor, broken suspension, no windscreen etc. , but the ISS is a much higher risk. If parts fail, there is a high likelihood that astronauts will die.
The ISS is old, and the amount of maintenance is getting quite high-the schedule for each astronaut includes essential maintenance tasks.