I agree indeedI see this argument a bit and it really doesn't wash with me....
Think how many new pets have been introduced since the beginning of MoP...literally hundreds and many with entirely new skins, not just a recolouring of old ones. They could easily have kept a handful aside for future rewards and it wouldn't have made one bit of difference in development time.
The players wouldn't have been disappointed with less pets in the world as they wouldn't have had a clue but they would have been pleasently surprised when anniversary time came round
Also, think of all the "toys" that have been introduced into the world during MoP - again...they could have held a few back for things such as anniversary gifts.
NOT arguing on the entitlement issue here just please don't use the "would prefer resources were used for future content" excuse as it really doesn't hold