I'm not so sure. I believe most people do realm transfers because they want to change the home realm for their characters, have had bad experiences on their current realm, or want a change from their current realm. I don't think most people who do transfers do it for heirlooms specifically.
What do you think is greater: the number of people willing to pay to have their heirlooms transferred to another realm, or the number of altoholics that will continue to subscribe knowing they can transfer heirlooms between realms for free? Sure, altoholics don't make up the majority of players but they still make up a fairly large proportion of subscribers and it's in Blizzard's best interest to keep that proportion happy. Implementing heirlooms as truly account wide would be such a move.