Heaters fail. It will happen. I've got a Marineland that has failed on. Just like yours. It's why I always recommend 2 heaters at just over half the wattage required for a tank. When the heater fails since it's a lower wattage it can't overheat the tank if it fails on. If it fails off the other heater can at least keep the tank from going too cold.
They give their products a warranty. If it's within that period I have no doubt they would replace the product. No company will replace the livestock though. How long after the warranty period expires should they have to replace it then so they "stand by their product"?
This is a good point. Frankly, for the price of heaters, it's probably wise to replace them every two years or so whether they need it or not, especially if you have the luxury of buying them on sale.
Next, heaters aren't something that I like to overkill. Putting too many watts in the water means that the heater is clicking on and off more. They're designed to turn on for an ideal amount of time and turn off for an ideal amount of time. Vary too much in either way and you run the risk of shortening the life of the heater.
Finally, I like an in the tank heater or heaters rather than inline ones. The indicator light will let you know if it's coming on and turning off. Additionally, in my experience, failing heaters often lose the light before anything else due to moisture invasion. A flickering light is a sign that it's time to ditch the heater and buy a new one.