I remember the outrage quite well when Facebook started spamming ads to the phone numbers of people who were forced to give Facebook phone numbers for "security" purposes and promised to never been shown ads on those numbers. Or when Jack Dorsey's Twitter account was hacked because of SMS 2fA.
Last, phone numbers are general identifiers used in the search boxes of various data collection tools. Maybe you can search by the Threema ID as well, but that requires the tool to be a tiny bit more sophisticated, and that means the people who like to invade privacy of are a bit more frustrated.
That isn't "smartness signalling" or whatever, that's a real concern.
The phone number was the only thing that was preventing them from keeping metadata such as usernames on their servers(until they introduced the PIN). It was a tradeoff between two concerns.
Last, phone numbers are general identifiers used in the search boxes of various data collection tools. Maybe you can search by the Threema ID as well, but that requires the tool to be a tiny bit more sophisticated, and that means the people who like to invade privacy of are a bit more frustrated.
That isn't "smartness signalling" or whatever, that's a real concern.