I agree, but that's why I labeled it a prisoner's dilemma. If literally everyone ran an exit node, then if prosecutors wanted to assume that any exit node facilitated the transfer of illegal material, then they would need to find other ways of proving criminality other than what packets were sent from your IP address. They can't seize everyone's hardware. If they wanted to obtain a warrant for you, they'd need more probable cause than "he's running an exit node" (because everyone runs an exit node).

As it stands, there's already a certain level of injustice, because corporations like Google and Microsoft facilitate all sorts of illegal communications, and the worst that happens is they get a letter from the feds asking them nicely for their subscriber's information. The investigators don't jump to the conclusion that the CEO of Google is a child predator and seize all the Google servers. But for an independent system admin on a home network, that's exactly what they do, even though there's no fundamental difference other than the size of the operation (and the implicit assumption that exit relays are unusual, which is the unfair assumption I'm trying to draw attention to as an explanation for lack of plausible deniability on the part of the idealistic sysadmin in a world where exit nodes are unusual).