What would they get? Best case, there is a contact name associated with a phone number they think is suspicious? Wouldn't that have been backed up on iCloud?
Also, what's the endgame? They want to convict the dead guy? They want to convict the neighbor who sold them guns, maybe?
You think so? It seems disingenuous for you to suggest that when all the evidence points to this phone having nothing useful on it, that this is just a thing the FBI wants to have done for precedent. The FBI has been crowing for backdoors into all our private communications long before this phone became interesting.
We dont know what they found in the lake.