Wait, what? Who said anything about stealing? Publishers might try to argue piracy but CDL doesn't seem to fit that either on account of the DRM.
Honestly I'm not at all clear what a cohesive argument against CDL would look like. Given the first sale doctrine it wouldn't even surprise me if some of the current ebook pricing practices were actually illegal.
The argument is simple. Copyright allows the copyright holder to enjoin anyone else from making a copy of a work - including a digital copy. Essentially all software works by making copies of data that is uses.
The first sale doctrine doesn't apply (IMO) because it sets limitations on the rights of a copyright holder to a particular copy of a work after it has been sold. Each "loan" of an ebook is a fresh copy.
If you look at the Controlled Digital Lending website[1], they pin most of the legal legitimacy on fair use, not first sale doctrine, which I think is probably much more solid legal footing.
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The EU has already done this since the ECJ ruling in 2012, and the US is behind the curve.
That being said, it seems like the ultimate end run around otherwise well established law surrounding books. As you noted essentially all software works in this way. Throwing out all established laws and end user rights "because digital" hardly seems reasonable to me.
How does such a line of argument square with the fact that even making personal use of a digital item inevitably involves multiple copy operations on a local device in the best case? Even moving it between two partitions on the same device involves copying it. (I suppose I should just be grateful that the digital overlords saw fit to grant me permission to move things between logical volumes. /s)
Most digital goods are "sold" as a copyright license.
1. After some quick searching, I don't think CDL has been ongoing for anywhere close to "decades".
2. Some IP like trademark can be lost if it isn't defended. I do not believe that is true for copyright.
3. In general, this is not the way law works. If you steal from someone for years without being caught, you don't create a right to steal from that person over time because it's "been ongoing for anywhere close to 'decades'". The lone exception that I"m aware of is land ownership - adverse possession is a thing, but it only applies to real estate and it applies to a specific piece of real estate, not a pattern of activity.