Gwyllt
Perhaps people can talk about shows that have not had this problem
I buy shoes and boots from Ecco, Hotter and Josef Siebel.
The only shoes (or boots) I have had this problem with are the ones which had been stored for a length of time in a fitted wardrobe the back of which was on an outside wall, where damp had got into the soles breaking down the polymer chains as described upthread.
I have never had a pair or shoes or boots that had regular use all year round do this. They are usually retired just because they have become scuffed, faded or stitching has become frayed.
I keep four pairs of tatty old shoes, different brands, by the front door, the doors to the garden and by the door from the utility room to the garage, for slipping on as and when. They all get some use each week just pottering about. After many years, the soles show no signs of crumbling.