This is a good question: I bought the 5E rules to learn how to play the game. At this point, I have my 5E books put away while I'm actively playing a campaign. What I need are the character class rules, and they are online. I just leveled up the character I'm playing in Roll20 and I didn't need to crack a book.
When we get the next edition, if the rules are the same, I can play the old classes, and the new class material is online ... there's no need for me to pick up the books. That's the question I have: what's different and what's going to be the same? I initially thought there would be some significant rules changes to the game to prompt me to pick up the books. I'm still convinced we are going to see some of that, really, but WotC isn't showing much of that. Yet. If that's the case, I'd pick up those new books to learn the game. If that's not the case, if there aren't any significant changes to game play, buying the books is a harder sell for me. That's why it matters to me.
I still buy a fair number of rpg books and materials because I'm actively engaged with the hobby and care about it, not to mention having some friends who I can support by buying their books. Those are games I feel I have stakes in. I don't feel that same connection with WotC and haven't felt it for a while. In the past, new books would have been a no-brainer for me and I would have ignored this discussion. Now, I consider "do I need this to play the game? What am I getting for it?" And so I'm here discussing the edition.