That's not a problem. Sure, it takes time, but why is that a problem? New NPCs for deliveries will likely be added, but if past history is anything to go by, they will be added at a slower rate than the achievements for the old ones can be finished.
It may take several years to finish all those achievements, but since players typically stick around with the game for several years, and since the achievements aren't actually needed for anything else, this is not a problem.
