Mumben
I am amazed that you have had shoes that long and expect them to be as new! If I’d had a 15 year old pair of shoes that had deteriorated, I would just throw them away, I would not have thought to write to the company to complain!
I have shoes much older than that that are absolutely as new.
I used to have a bit of a shoe habit (I'm over it now that I can't do heels for more than an hour) and have loads of pairs of occasion shoes that have only been worn a handful of times, sometimes only the once. They really shouldn't disintegrate when stored, and it's only happened to me once.
£70 wasn't cheap for shoes 15 years ago. I don't think I'd have had the courage to complain, but why not, if they'd been barely worn?