This is actually not without a real-world precedent. Checker Cab was a well-known manufacturer of taxis in New York City throughout most of the 20th century. Its iconic exterior design remained virtually untouched for nearly 40 years between the WWII era and 1982, when fuel economy restrictions finally forced them to upgrade the whole thing, inside and out.At least they aren't repurposed DSC shuttles, like the school buses and taxis.
Did PIC reuse DISCO models? Yes.
Are they a 23rd century design considered to be just as iconic as the Checker Cab (they are also yellow, after all) in the late 24th century that they felt the need to preserve it (perhaps for nostalgia's sake), just upgrade the interior to coincide with 24th century technological advancements? Also yes.
