A case of no one taking ownership of a problem and solving in a fair way

Very unhappy with the service received by Hirschs hillcrest and Defy. Bought a Defy gas stove and oven about 4 years ago 2 years later the oven thermostat packed up after some struggle managed to get a new thermostat cost about R125.00. 2 years later the same part failed only to be told the model is discontinued and no spare available. After pressurised they offered a part to me for R800.00. The question is. Is an appliances life span only 4 years and will i need to spend R800.00 every 2 years for a replacement thermostat. Is this what we should expect from a reputable brand and dealer