I guess my point might have gone a bit over your head
If I sue you for patent infringement, I'm claiming I own a particular idea, and that you're using it without my permission.
Your potential defense is (obviously) that
a) You're not actually using the idea in question, (your method is not covered by my patent) or
b) That you actually have my permission (it was included in some licensing agreement, etc and I'm just confused or being a jerk) or
c) I don't actually own the idea (my patent is invalid).
Now, it seems pretty self-evident to me that if I don't actually own the idea (my patent is invalid) you should be able to point that out in court.
Now, you make the argument that by allowing you to claim my patent is invalid in court, you'd be placing the burden of proof on me to prove that my patent is valid. This is the common, natural way in any other kind of lawsuit - the plaintiff (the person making the claim) is supposed to prove their claim is true. I don't see why patent holders should be privileged by the assumptions that they were truthful to the USPTO, and that said office conscientiously only issues absolutely watertight patents. That last one in particular is utterly laughable.
You also claim that this would make suing people for patent infringement more expensive and defense easier, to which I say, that's a huge point in favor of my argument.