Disintegration of Tyres after only 7 years

My Michelin tyres have started to perish on the side wall, however the tyres has still 4mm left of the original 7mm. I have raise this with three people at Michelin including the rep, Techical manager and sales manger who have hid behind a 5 year warranty however they tyres are only 7 years old with low mileage and are busy disintegrating. Worst service and brand won’t purchase ever again