Right, but a patent X can only be used to attack technology Y if X predates Y. So once all the pre-Y patents have been struck down you're a lot safer. Even though X might be a totally unoriginal patent, if it predates the technology it's attacking, it might still be hard to defend against. Strike down the patent and it's done - and you cnan't assign another patent in the past.
Technologies continue changing. Your core technology may be older, but if you've integrated with some recent Z, you can still be attacked by patent X, even though your core technology is older.
Look at Apple for instance. It is older than all of the patents it is being sued with. Many of the lawsuits that it faces are for cases where the patent is younger than Apple's technology. Apple still gets sued for it though.
