2013 BMW 328XI Wheels Problems

I PURCHASED A 2013 BMW 328XI CERTIFIED PRE-OWNED SEDAN 6/22/16 FOR MY WIFE. IT CAME WITH 18" WHEELS AND PIRELLI CINTURATO RUN-FLATS. THE TIRE-PRESSURE MONITOR (TPM) WARNING STARTED COMING ON A FEW MONTHS AFTER WE BOUGHT IT, SO I PUT AIR IN THE TIRES. BUT IT STARTED HAPPENING MORE FREQUENTLY. I TOOK IT IN FOR SERVICE. THE DIAGNOSIS WAS THAT THE PASSENGER REAR WHEEL WAS CRACKED AND LEAKING AIR. I ASKED IF THE TIRE WAS DAMAGED AND THEY SAID THERE WAS NO VISIBLE DAMAGE BUT THEY RECOMMEND REPLACING IT ANYWAY BECAUSE THE WHEEL WAS DAMAGED. I ASKED WHAT WOULD CAUSE A CRACK IN AN ALLOY WHEEL, AND THEIR ANSWER WAS THAT SHE 'PROBABLY HIT A POTHOLE'. I COULDN'T BELIEVE THAT WAS ALL IT WOULD TAKE, BECAUSE IT'S IMPOSSIBLE TO AVOID HITTING POTHOLES ON OUR FREEWAYS. MY WIFE DRIVES VERY CAREFULLY AND ONLY PUT 5,700 MILES ON THE CAR IN 14...