You underestimate their potential
Kid and law along with their crews are obviously not comparable to any of the emperors yet but that's not the point. They still the 2 biggest pitchers of the new gen outside of luffy and will keep on growing stronger even after losing to shanks and teach.
Oda didn't build them up since shabaody and have them participated in taking down an emperor in wano just to lose and be forgotten
Yes were in the ending part of the story but its not where kid and law stories end. Law is still gonna learn about the will of d and kid will still prove himself by the end of the story