"Do you have no other way of investigating these criminals?"
"Could you not put an officer out to surveille them?",
Your argument kind of falls on itself there --> We can surveil them in person, but not digitally?
Why?
I think the tech crowd has this wrong.
The issue - as you have indicated - is not 'By What Means To Surveil'.
The issues are: Legitimacy, Proportionality, Oversight.
Messages, tech, sign language, in person, phones or messaging pigeons, the issue remains the same:
Is there a legitimate reason for access?
Is the intervention proportional to the probability of cause, the ostensible crime, the risk to other citizens and the public good?
Is there authoritative Judicial oversight of the surveillance, and, is there sufficient Congressional oversight of the legality of the program?
Those are the questions.
Should police be able to tap Signal (or anything else) for data on anyone they want, for whatever they want, willy nilly without a Warrant or oversight?
Definitely not.
Should Apple be scanning content for crimes?
Probably not, but that's slightly more complicated.
Should the police be able to access the Signal messages of someone they apprehended at a murder scene wherein other suspects fled the scene, and are therefore likely the suspects accomplices?
Likely yes. Or at lest, most people would agree with it both in the pragmatic sense, and also the Constitutional sense.
Should the FBI be able to, with special permission of a Federal Judge, watch all cell tower transactions in 5x5 mile grid grid, while there's a literal manhunt on for literal terrorists during a literal state of emergency?
Probably yes again, it's hard because the proportionality and tactics are rare and unique.
The technology is a secondary issue.