Whilst they are overkilling with the Arceus, I think they have to because they have nothing else to make a set off.
Between R/S/E and D/P, The card people made the Delta Species, because all the other sets had based off games, they had nothing to base off until D/P. The final set, PK was designed by America itself because they had no D/P yet, whilst Japan did.
With Platnium, they've made 4 sets, including this one. They're out of stimulus to make the set...so they'll focus it on Arceus, the last significant thing left in the D/P arc. Next set is probably going to be based off the HG/SS, that or something else.
That or they REALLY love Arceus.