Except that unlike most games, 3.5 has free & legal online documentation. It doesn't "die" just because the books run out. I have players in my games right now that run characters off of d20srd.org, and have never bought 3.5 edition books.The issue is it's a market that can only shrink. Eventually people will stop playing, and it's hard to bring new players in without the ability to buy rulebooks. Developing for 3.5 doesn't make much sense. Pathfinder is at least supported. It might be possible to do an adventure that's compatible with both because of their similarity, but publishing adventures for a game that's no longer printed seems like a losing proposition.