Your scenario would specifically require the cops to ask their techs for a detailed report and then deliberately lie about it's conclusions to attack completely random people, and just FYI the last few rounds of this happened when public WiFi was new and the cops kept losing so badly in courts that this doesn't really happen much anymore. You don't even need a great lawyer, just an average one who can find the precedence.

There's no "additional fingerprints" of relevance binding any node in a tunnel to the communications in the tunnel. It uses PFS and multiple layers of encryption (tunnels within tunnels). They need to run a debugger against their node to have any chance to really argue that a specific packet came from a specific node, which also would ironically simultaneously prove that node didn't actually know and was just a blind relay (just like how mailmen aren't liable for content of packages they deliver).

Your argument is literally being used to argue that nobody should have privacy because those who don't break laws don't need it, yet you yourself are arguing for why we still need privacy if we haven't broken laws. The collateral damage when such tools aren't available is so much greater than when privacy tools are available. One of the greatest successes of Signal is how its popularity makes each of its users part of a "haystack" (large anonymity set) and targeting individual users just for using it is infeasible, protecting endless numbers of minorities and other at-risk individuals.

In addition, it's extremely rare that mass surveillance like spying on network traffic leads to prosecutions. It's usually infiltration that works, so you running an I2P node will make zero difference.