Unfortunately, the way patent suits work it could be enormously expensive to defend something like this, even when the patent is clearly bad.
You'd be arguing that the patent is invalid to start with, but the court would probably start from the position that you are actually infringing a valid patent (it was granted after all), and grant an injunction to prevent further harm ("stop giving people the software until we can work out if there is any merit to your claim that you aren't infringing"). You then need to put together a case to show the prior art, and you can bet that they'd contest every single point. This whole process could take years, and cost hundreds of thousands of dollars that you won't get back even if you win - there isn't really a provision to recover costs in patent cases because there is the assumption that every claim is made in good faith
That's probably an impossible task - getting enough people who are experts in every possible field enough to judge novelty and innovativeness wouldn't be feasible.
An alternative is the way the Dutch assess patents - they don't, and grant them automatically on filing, but that means you remove the assumption that they are valid on their face if they get challenged