Not sure I can answer all those accurately.
What I will say is that, as with many mass produced items, it is likely that not every one made will be exhaustively tested, although I would expect some form of batch sampling. Many modern manufacturing processes look at controlling the input materials and the actual assembly process. The theory is that if the materials are right and they are put together properly, then the finished product will be OK. This works for large quantity production where the product definition is good and there are little variation from item to item.
Because this relies on statistics, it is inevitable that some less than perfect items get made delivered and used. Obviously, some end user finally gets this and experiences this quality issue as an item that has less than expected life.
I can't say for sure, but I wonder if this is what you have experienced. Perhaps some weakness in a seal let hot water into the electrics, thus corrupting the sensors output.
In terms of cost, again I can't say for definite, but I wouldn't be surprised if the part itself was £75 - £100 with up to 3 hours labour needed to diagnose, drain the coolant, replace the part, re-fill the system and check all the seals are sound. Depending upon where you are in the country labour charges may vary bu at, say £75/hour, the total could be in the region of £300 to £400.
Manufacturers would consider this acceptable on the basis that the total quantity of faults is small (because of good control of materials & process), so is cheaper overall than fully comprehensive testing on every item made.
Regarding faults that occur outside the warranty period, well - we knew what the terms were when we signed on the dotted line. It's all part of a complex set of questions relating to cost competitiveness and overall product quality.
