Apart from just not having encrypted data, the only way to achieve what you're suggesting is with a government backdoor into the encryption.
Any backdoor - any! - will result in your data being exposed, sooner or later. Your Signal messages could then be exposed in a data breach on the dark web for all to see.
It is not worth it to risk everyone's privacy to allow for the chance at easily prosecuting a small number of crimes. Remember - you're not preventing crime this way, just allowing for easy evidence capture. There are viable alternative ways of investigating crimes, as others here have said. There are not viable alternative ways of protecting our data.
Any backdoor - any! - will result in your data being exposed, sooner or later. Your Signal messages could then be exposed in a data breach on the dark web for all to see.
It is not worth it to risk everyone's privacy to allow for the chance at easily prosecuting a small number of crimes. Remember - you're not preventing crime this way, just allowing for easy evidence capture. There are viable alternative ways of investigating crimes, as others here have said. There are not viable alternative ways of protecting our data.