Your argument makes sense, but does not account for a lot of player's budgets. For many paying a smaller monthly fee is doable, while larger sum purchases are not. Most have to account for bills and other life expenses before any frivolous purchases and a modest subscription model is a better alternative. The downside of not having the material permanently is acceptable in that case.
Will there be a subscription plan to use the content without having to buy the digital books? I already own the books physically, I don't need to own them twice. I just want to use the service and I wouldn't mind being on a subscription for it (kind of like MMO subs).