Well, first, they are customers, and WOTC has to ask whether 10 buck every few months is worth nothing all the time. Will the 10 bucks every so often people join the full program or quit? Time will tell.
But we, as always, have to realize that DDI has lots of customers that are not here, and are apparently not on WOTC's forums either.
You are looking at the cost equation from a vary narrow focus here.
The cold hard truth is not all customers are equal, nor are all customers necessarily desired. If
all customers were the only desired outcome then what you posit would be
material to WotC's business strategy for D&D.
On the other hand development costs (to both create and maintain) a service (DDI) in support of their main product (Printed D&D) have upsides and downsides.
It's not just a mater of anti-piracy or a constant revenue stream that WotC is moving their service into the cloud. The cost of designing stand alone products that contain elements of your database and distributing them to a lot of different locations is inefficient and wasteful. For the size team they have having all the data in one place (one DB) vs. multiple locations reduces the complexity of their task.
Imagine having to create 4 or 5 programs, each with their own database, that have to work flawlessly together in order to interact with one another. If the databases shared any data, they would have to be updated separately (meaning you enter the data multiple times). The users have to make sure all programs are updated concurrently for them to interact.
Now imagine having a single location for all that data (no duplication of the data) and you just have to design a user interface to interact with the data. Further any information created using the data (characters, monsters, traps, new feats, new powers, etc.) is in this single location. I want new functionality, I design a new UI (or update an old one) that interacts with the data.
There are tons of reasons to move to the cloud data model and the benefits of it probably outweigh the continued patronage of those that have been getting an out sized reward for intermittent revenue that WotC gets from them.
The fact is, they only need a few folks that subscribe twice a year to commit to a full year to make up for the ones that leave (1 customer adopting for every 5 that leave).
I am going to bet that WotC's plans for increased functionality of the DDI tools means they believe that more people are going to want the tools. A sneak peak is easy to do, just offer a free weekend were folks can come try them out (no more brain dead and outdated "free" version of CB) you can show them the whole ball of wax all at once. Then close the wall and see who comes back with dollars in hand.
So, in short, yep WotC isn't stupid in regards to costing out their products. They know what targets they have to hit and for the most part they can hit them.
Unfortunately their communication of why these choices they made are better in the long term for customers is pretty dismal. Instead the narrative is about people's fear and anger at the changes. They could have done a hell of a better job of this and they haven't.
My two coppers,