First, it's an assumption with no evidence that they make more selling a book on DDB, not a fact. Unless you are privy to their books, of course. Running servers and keeping them online is not free.
Second, I would think that they want to sell you both a physical book and digital. Heck, they'd appreciate it if you bought a second physical copy with the special cover as well just to put on your shelf.
Third, profit is profit. There's very little extra overhead to setting up the books relatively speaking.
Fourth, we have no idea how many people would reject online only. People keep floating this hypothetical future where people won't care. Which, if they do, why would it matter? This is a confusing part of the whole argument. Online is going to be so amazing we'll not want books. So if no one wants books, which again I don't see happening, then no one is affected if books are unavailable.
Fifth, the assumption that they would not lose significant percentage of new and casual players. That if you cut off books, which many people love and prefer, that a competitor won't come in and fill the gap.
Last, but not least, with DDB you have far more resource sharing than you do with physical books. You still have the physical book setup overhead but it's much, much easier to share my DDB resources with my entire group. It's a worse model for them than selling books.
I currently share my DDB account with a dozen or so people at this point across 3 different groups. All those people have access to books that they now do not have to purchase for themselves. I'm not convinced that they would not be making more money if the option to ignore digital entirely was open to them.
The thing is that I think they are practically being forced into digital whether they like it or not just to stay relevant. This is not something they were at the forefront of, after Gleemax crashed and burned they were understandably reluctant to pursue a digital approach. Now that they finally are, they're being pilloried for ... wait for it ... pursuing a digital approach. They can't win.