I had a feeling this wasn't too well thought through.

It makes it easy to claim another person's identity, while at the same time, well-known people likely wouldn't want a username that makes them easily approachable to strangers that otherwise woudln't have their number.

I am not a known person but I still have no incentive to pick a username. Someone wants my number, I'll give it to them.

For a medium like Twitter, usernames are an obvious feature. For DMs that were previously number based, not necessarily.