are that your new monitor failed within six months of the date of purchase, and therefore under the terms of the supply of goods and services to consumers regulations the fault is deemed to have been there on the day you bought it - unless the retailer can demonstrate otherwise.
Your contract was with Misco, and it is to them that you should have looked for satisfaction, not Iiyama, who will only act within the terms of their manufacturing warranty.
Unfortunately the waters have been muddied by the fact that you've accepted replacement monitors - you no longer have the original with which to press your case. In the circumstances you're a bit stuck, and my best advice is that you press Iiyama really hard on this. They have a reputation for making superb monitors, and in the circumstances I feel they owe you something better than a 2 or 3 year-old monitor. Get a bit pushy - but not in an aggressive way.