I would say usefulness and how much money you're blowing on it, and I'll use myself as an example. I read books, mostly nonfiction histories, and I get rid of something if I don't like it at all. I buy all of my books used now to save money. I've made a vow that my bookshelves will cap at 145-150 (not counting situational or seasonal books), and I will start getting rid of stuff if I get over the 145-150 cap. If you look at "bookshelf tours" on Youtube, you will see people, mostly women, who have book collections that get up in the THOUSANDS.
Like is 2000+ books practical to you at all? Do they resonate with you personally? Do you care about what point the author is trying to make? Are you interested in the topic or genre? Do you even have the TIME in day to day life to read 2000+ books more than once? Do you seriously feel the need to spend 50 to 100 bucks on a Super Ultra Mega Hyper Deluxe Illustrated Edition of a standalone fantasy novel you like? I mean come on, this is just mindless consooming. You're not getting anything out of these books, you just want to cram a bookshelf full of YA schlock.
