I am outlining a hypothetical scenario where the authorities choose to look through my phone for ${UNRELATED_REASON}, and the presence of Signal's hypothetical exploit files would cause their digital forensics software to crash. In this hypothetical scenario, that crash in and of itself could then be taken as a reason to charge me. I don't believe "the app software maker selected my phone at random to have those files on it!" would help me out in that situation; that's the sort of nuance that tends to get completely lost.
The opt-out option would help me because, having opted out in advance, the hypothetical exploit files would not be on my phone.
I do agree that this is a slightly far-fetched scenario. However, I am very confused how HN can generally be of the opinion that having unwanted malware on your phone is a good thing, even if Signal put it there.
This fantasy that you can get away from a situation in which you are being interrogated and your phone is being searched by being compliant and proving your innocence to the nice police officers is extremely dangerous. Being compliant and providing information beyond what's legally required is the exact opposite of what you want to do, and preventing yourself from being charged should be the last thing on your mind. You should take it as a given that you will be charged.
edit: If you're outside the USA ignore this advice and listen to someone more local who knows their stuff.
Why would they make it crash the software though? There's no reason to do anything that's visible to the operator, perhaps it just alters data previously downloaded from other devices and then deletes itself. Or changes nothing other than adding a friendly file called moxie.says.hi.txt