You raise a good point Mick, but it is about how the tyres are stored when off the road.
It is accepted by those in the industry, that once tyres reach a shelf life of around six years, depending on how they have been stored, the rubber could be compromised. In my own case, because I store my winter wheels with the tyres on the rims in the garage, inflated to the correct pressures, in the dark with good ventilation on a wheel tree and properly covered, I personally don't have any particular concern because I know what conditions they are being stored under.
The people who should worry, whether their vehicle is fitted with summer or winter tyres, are those that have had the same tyres fitted to the car for years in the case of those doing very low mileages, or those taking the risk of buying part worn tyres, something, personally, I would never ever do.
Look at the sidewalls of very old tyres and often you'll find cracking, one sure way of telling that the rubber is breaking down and often, they'll be the cause of blow outs at speed on motorways. Caravan owners are often the most guilty of this, years and years using the same tyres and then, bang, blow out.
I accept that there are plenty of people on this site who think that those of us who choose a bit more safety over convenience, or are just being anal, wouldn't do what folk like me do, but then that's our choice and I'll spend my money how I like. Not only that, if I do feel like taking a trip over to Germany or Austria in the winter, I don't have to worry mucking about sorting out tyres as I already have them!