This doesn’t prevent people from using products they bought in the last year in any way. The two books out still stand.
While there is a pandemic at the moment, these rules will last for a long time to come (at least a year) during which time we might be able to get out and start playing games again. And, as you suggest, there a business reasons for doing it, too - nothing wrong with that.
3 books is a maximum. You don’t have to use the magic rules (or even buy the book). The same applies to uncharted empires - unless you’re fielding one of those armies, you don’t need the book to hand. They’re optional in their use.
Finally, Tthere was a point during Kings of War 1st edition where you could be using the core rulebook, Kings and Legends, and the Basilean Legacy at once, not to mention Hellfire and Stone. As I said, it all depends on whether or not you choose to use those rules. I can’t really comment on what you feel about this, but I can offer facts that knock through the ol’ rose-tinted vision.