Bridgestone potenza runflat tyres shredding

Hi<br> I purchased a new set of run flat tyres (Bridgestone Potenzas 255-40R17 for my BMW 320d touring in 2015. I have driven ?20000 km on the tyres during the time to date. My tyres treads are still fine however , my rear left tyre is beginning to shred. The rubber is pealing of whole and the steel in the tyre is exposed which leave me feeling very unsafe, thus I have stopped using my car until I can replace all my tyres which is a costly affair.I am just wondering , do all bridgestone potenza run flats ware out so quickly and have so poor quality.