Not only was the fleet of 747s not grounded until the exploding Center Wing Tanks fixed, the NTSB's final order on INERTING fuel tanks on commercial aircraft was issued in May 2008, twelve years after the TWA-800 disaster. It required all new US commercial aircraft to have auto-inerting of the empty volumes of fuel tanks after 2010 (!). However, it also requires that specific existing commercial aircraft must be retrofitted with inerting systems by 2018. Strangely, of the several thousand Boeing 747s manufactured before 2008, only a mere 60 were being required to be retrofitted with inerting devices. That tells me that it was NOT a major safety issue or even a priority.
How many times do I have to tell you that until a few years ago, inerting systems were too heavy and expensive?
As far as older 747’s being exempt, if that’s true, maybe they’re being used as freighters. Very few, if any, older 747’s carry passengers anymore. Plus there were other Airworthiness Directives that made the older 747’s safer. Fuel tank explosions are rare. I don’t think anybody’s disputing that.