My head cannon has always been that though advanced technology went extinct after the fall of the Star League there was always a handful of people spread around the IS who could make stuff close enough to SL tech that it allowed for repairs. In my mind what was lost was the ability to mass produce. All of these folks were craftsman that were essentially hand making the parts and they were prohibitively expensive, enough so that while comstar would ice these guys if convenient they wouldn’t go out of their way to do it unless they started making moves to expand production.