said by James_C:Sorry but you are simply wrong. You're taking an overly simplistic idea that all a manufacturer would consider is replacement cost when this could only be true in a closed market where there is no competition.
A manufacturer first has to make the sale, either making good money because of their reputation, or setting for far less because everyone considers the product junk.
I would echo AVD
's question about what century you live in. Price (not value) is the primary factor in most purchases today. If quality was a primary factor for most purchases products manufactured in foreign countries using inferior parts and inferior labor would not have such high sales in the US. For items such as water heaters, for instance, if two choices have the same warranty length but one is 20% less expensive than the other, which do you think will have more sales?
I am certainly not taking a simplistic view, and in an earlier reply I even explicitly stated it is not a simple calculation.
This discussion has gotten far off topic from the OP question so lets get back on track.
The short answer is that 18 years is a good lifetime for a water heater. If an inexpensive repair or part replacement may be made that would be an option but for something this age spending a large amount of time or large amount of money on a repair likely is not worth it.