Hi David
You make a very good point but the publishers have already thought about it as they want to tie buyers in by committing to a subscription for the entire collection.
As already pointed out the publishers are running a profit making business and wouldn't commit to actually publishing unless they either very sure of getting a good return or have enough advance subscriptions to cover their basic costs.
I presume that the potential profit is very high as I can't see the model itself having a huge production cost.
Paul