Warrants to hack devices are a lot less common and generally harder to obtain. That's why police will send Google warrants for "give us info on every device who has been in a radius of x between y and z time".
I'm sure Apple did their very best to protect their users, but I don't think their very best is good enough to warrant this kind of trust. A "secure cloud" solution will also tempt future projects to use the cloud over local processing more, as cloud processing is now readily available. Apple's local processing is a major advantage over the competition but I doubt that'll stay that way if their cloud solution remains this integrated.
Your example indicates a situation where law enforcement does not know which device belongs to their suspect, if they even have one. That's a very different scenario from a targeted "tell us the requests belonging to this individual".
Warrants to search a device are extremely common place, otherwise the likes of Grayshift and Cellebrite would not be around.
From a threat modeling perspective compromising PCC is high risk (Apple's not just going to comply and the fight will be very public, see the FBI San Bernardino fight) , high effort (Long protracted court case), low reward (I only see requests that are going to get shipped off to the cloud). If I were law enforcement I'd explore every other avenue available to me before I go down that particular rabbit hole which is exactly what this design is intended to achieve.
iOS won't send any data to a PCC that isn't running a firmware that's been made public in their transparency logs and compute nodes have no way to be debugged in a way that exposes user data[1]
And at the end of the day, this is going to give the warrant holder a handful of requests from a specific user? Why wouldn't they use that same warrant to get onto the target's device directly and get that same data plus a ton more?
0: https://help.apple.com/pdf/security/en_US/apple-platform-sec... 1: https://security.apple.com/blog/private-cloud-compute/