I think that it's also a matter of how much more would it cost to fix a non-serviced movement compared to servicing a working movement. By cost I mean money, time and effort.
Let's assume that you want to keep the watch for at least 5 years.
So, taking preventing measures you would pay the money and hassle no matter what, and probably enjoy it for the 5 years. Still, you have a chance, let's say 10%, that you would have some problems that you would have to fix.
Taking the risk route would mean that you do nothing until something is wrong. Let's say that there is a 70% chance that it would happen considering the poor QC and assembly conditions.
Now, X is how much that a preventive service would cost, and Y is how much that fixing a non serviced movement would cost.
Preventive route cost: X + 0.1*Y (10% probability that something would go wrong after service).
Risk route cost: 0.7*Y + 0.1*Y = 0.8*Y
Now, obviously I made some rough guesses here, and some would say that peace of mind is priceless.. But, at least for me, I'm probably in the ball park.
So, if trying to calculate mathematically, it all comes down to how much would a fix (Y) cost compared to a regular service (X). Here, the experts could chime in. My hunch is that the difference isn't that big. The fixed costs are much greater than the variables, and are pretty much the same in both cases.
According to my rough assumptions, only if fixing costs are more than 150% higher than regular servicing, than, financially, it would be wise to go the preventive route.