That is absolutely not a library's main goal. A library's main goal is to provide access to information, not preserve it. Archive.org is thinking about what an archive is for. That's is understandable, but don't let their confusion confuse you.
cf https://www.britannica.com/topic/library/School-libraries
I'd say you're both wrong, and both right: "library" doesn't imply either, in and of itself it's just a repository of material.
Where it leans in its goals depends on the type of library: a national, reference, or research library will likely have preservation as a foremost goal (the first and second especially, but a research library needs to preserve primary and secondary sources no matter how old they are, unless they get punted to a specialised reference library), while a public or academic library will be more concerned with having resources patrons are generally interested in.
Maintaining a collection another way of saying preserving information over time.
So I would argue IA is correct.
But if your local libraries are anything like mine, the focus is far less about books and more about providing a community space.
This would be a concern for a centralized archive.org I would hope there is a great backup strategy in place that can prevent deletions from the archive
Even a rare book store would glady empty their entire inventory for a price.
In my opinion it comes down to the purpose and reason for existence.
To answer your question I don't think selling a single book makes a library less a library. But if a public lending 'library' had a price tag on every book, I would say it is a bookstore with loans.
In your mind do you consider an iron ore bulk carrier in Australia different from a natural history meusum in France or an iron ore dealer in Argentina? They all store rocks and minerals, including iron ore, and so they could all be called "iron ore storage facilities".
But only one of them has the purpose of studying and preserving artifacts and so it makes sense to create a subclass and call it a meusum.