B-21s would seem to bring little advantage to hitting Iran. Aren’t they smaller with less payload than B-2s? Iran isn’t going to be able to defend against anything stealth for a long time.
The B-2 rollout was in November 1988. Bush 41 was President-elect and stealth was just coming of age. The B-21 may not be overdue but I think it will eventually have a deserved place in the inventory.