Like they said, it's a replacement prototype for the M-42 Duster, and probably the last major gun-based AA system. Everything gunwise since has been pretty close range, operating under an umbrella of missile fire instead of being the primary defense system. Not to knock things like the M163 VADS but it's a low-and-slow killer, rather than a general all-purpose AA unit.
As for the T249, well, let's just say that the M247 Sergeant York beat it. And you know what a wonderful platform THAT was. Still, there's something that brings a tear to the eye when you say "37x219mm at 3000rpm" but maybe that's just me.
Sleep well, little BRRRRRRRRT. Sleep well.