I dunno. They would have to be rusted out or broken to be beyond service. The cover comes off, exposing the gears and post. A cleaning of the dried out lube, then a shot of TriFlow, light oil, lithium grease, something... would have them at least functional again. Perhaps the brass gear might show wear.
However, I'm of the ilk that you might as well install something better. Those tuners were chosen for kitsch and economics. It's almost expected that they are replaced at some point.
My equally poor quality, '98 era MIC 'pre D' tuners look to have the same hole pattern as the 'D' tuners. I didn't try to find direct drop in tuners, as most anything that would line up with the mounting screws and use the same push-in bushing would still be pretty poor quality. I find that Kluson style tuners work great. The locking Hipshots now on my '98 1959 U2 have mounting plates that are shaped like a Kluson. The stock tuners on my '18 Dead On Longhorn are Kluson copies and are working great.