> Instead, they linked the number of digital copies to the number of physical copies they had, effectively giving them another way to loan out books they had already purchased.

That's what they had been doing, for years. Publishers grumbled, but didn't file lawsuits.

It wasn't until the 'National Emergency Library', when IA decided to unilaterally ignore the 'controlled' part of digital lending, that the suit was filed.

Yes, but the lawsuit does not restrict itself to the NEL. The ruling against IA spends most of it's time establishing why Controlled Digital Lending itself is illegal, and then mentions "oh and they also didn't strictly enforce the owned-to-loaned ratio anyway."

My guess is that the publishers would have sued with or without the NEL; they were just waiting to accrue as many infringements as they could in order to force a quick settlement. If you sue immediately there's less damage than if you wait so you can sue for multiple infringements over the whole statute of limitations (3 years AFAIK).

The publishers didn't sue without the NEL; they had lots of opportunities, but CDL wasn't worth the fight to them. Shortly after the NEL, they sued. It's clear that the NEL was the triggering factor.

From publisher's/authors' perspectives, there is a massive difference between "controlled digital lending but we respect copyright" and "digital lending, and we can choose to dispense with copyright restrictions whenever we think it is justified".

One of those was an uneasy peace, based on the idea that IA - while it disagreed with the publishers on some issues - was at least acting in good faith. The other one would essentially have gutted all copyright protections, making respecting IP rights purely a matter of choice. Whatever you think of IP and copyright as a whole, publishers were never going to accept that massive change in approach.