prosfilaes
Adventurer
Instead of selling 3.5e, imagine what would happen if they gave it away. What if WotC declared most of 3.5e as Open Content, then revised the d20 SRD to include the wealth of material from other 3.5e supplements?
Would it hurt 4e? Would it hurt Pathfinder? Would Paizo be tempted to absorb the new information, thus releasing scores of new books and becoming bloated just in time for a simplified 5e to be released?
Ultimately, I think it's too late for Paizo to really use most of the new classes and prestige classes--they've moved in a different direction--but they could absorb some of the monsters, feats and spells. I think there would be a small, most negligible advantage to Pathfinder, though I bet most of the newly available material would be covered by 3rd parties instead of Paizo.
The question is, what does WotC gain from doing this? They didn't release it when the D20 license was a going thing, so why now? Given the situation, I can't see WotC being willing to work with Paizo on this, and even if they were, I'm not sure I see Paizo being willing to pony up the type of money WotC would ask for. (They did do a deal for the original Tome of Horror, Dragonlance and Ravenloft, but that was all in the 3E days.)