I go through cycles. Moves, fits of pique, and new editions have periodically prompted me to purge blocks of books from my collection. Most recently, I started working from home, which has caused me to reevaluate how I arrange my functional space. Last night, I actually ran my few remaining 2E books, a couple old Shadowrun core books, and Hero 5E to the Half Price Books. I'm debating what to do with all my New World of Darkness stuff that I bought about the same time 5E was announced (D&D won). The hardest, though, is the span of Dragon and Dungeon magazines I have, going back to the 1980s. I don't really reference any of them (occasionally Dungeon), but I don't really want to get rid of them. Those and my high-end Aces & Eights book, even though no one else in the group has any interest in westerns.
Still, I collect. I just have started collecting digital books. I'm slowly getting all the Eberron books from DMs Guild. I have a bunch of Fate stuff, in digital form. I don't really love reading and referencing PDFs, but it seems like the most likely format to be able to keep around, regardless of whether I'm actively using it, or not.