All tires will deteriorate at different rates in different climate conditions, storage, temperatures, ozone conditions, maintenance, etc.
General Motors used to test rubber products, hoses, etc. exposed to the sun and weather in Homestead, FL as this gave an accelerated deterioration rate.
You have no idea if the original owner used any tire products that are not recommended by Tire Mfg. that will cause damage.
As to your opinion of Michelin, I replaced my O.E. tires when they were 8 years old, that on two tires had hairline cracks just starting around the bead, the other 4 tires were like new, but since they were approaching the 9 year period that Michelin recommends replacing I did. My experience with Michelin tires have been very good, but I take exceptional steps to cover and maintain them compared to many users.
I my experience I find many problems contributed to the owners and their maintenance practices or lack of and not always the products.