That sounds like the definition of a public good. It costs money to create the good (defeat the troll), but once the good is created, there's no way to control

That sounds like the definition of a public good. It costs money to create the good (defeat the troll), but once the good is created, there's no way to control access to it (once the patent is invalidated, anyone can use it). A rational actor always wants to be the one who didn't pay to create the good, so only a benevolent, irrational third party will ever create it. The problem with a pact as you describe it is that joining is not mandatory, and a rational actor will just let everyone else join the pact, and the pact will never get formed. There needs to be an incentive for joining the pact, as well. Patent pools, ironically, may be such an incentive. I'm just glad that New Egg decided to be irrational this time around.