Couch that this not good quality

The disappointment is hard to handle when one spends a lot of money on an item. I purchase a couch from Hirshs, it was on sale about 18 months ago and the seats are now starting to sag and feel very uncomfortable and makes my back sore, I would have expected a couch to last way longer. I have asked Hirshs to replace this and they have said they will repair it! and it will take up to 10 days, my question is, was this couch was not constructed correctly in the first place?? and how will repairing it now after only 18 months ( I am the only person using it and its only at night to watch tv) help and secondly what do I do in the when the seats sag again after a fews months. I have suggested that they replace it with a different couch and I am very happy to pay the difference in price to get a better quality couch however it seems that Hirshs do not want to do that, I have purchased many, many items from Hirsh but this is the most expensive and should have lasted at leave 6 years, needless to say I will not recommend Hirshs for furniture.