I have some cups fae M&S or John Lewis or some of those places that have certainly lasted much longer than the Dons' one without fading, I expect it's something to do with the glaze and the finishing layers (I have a couple of M&S - we got vouchers - ones that have faded too). My folks have still got cups they use nearly daily that have their favourite child's (me) name on them that have been in use over many decades, so I suspect it's just the more general enshittification that modernity continues to serve. Back in my day, our lead cups would last for centuries. A bit of asbestos for a lid, kept everything nice and warm.