Exploring the Main
It comes to me that the Imperium was unique in that there are many worlds, all with different tech levels. Some had large economies, but at lower tech levels, probably because it takes a lot of capital to build a higher tech level, not to mention having to train all of the people to work there.
So, they had to use and maintain ships and other equipment at a level they could sustain. Not only that, there were ships out there a century or more old, and, by inference, other equipment that was also very old, but still irreplaceable.
That made for a large market in lower TL parts and tools, plus it also meant that the higher tech stuff probably was backwards compatible, at least for the next lower TL. This isn't always true, but it is for many components.
Further, depending on the part/assembly, lower tech parts may work, but maybe not as well. Take car tires as an example. You could put 1970s-era tires on a modern car, as long as the tires fit the rims. Rims would be trickier, due to the bolt patterns and the presence of disc brakes versus drum brakes. If the modern tires are low-profile or wider than the older ones, then it may not be possible to use the older ones.