Just seconding this and adding that, unlike the old DM endpoints which were pretty close to realtime, this new endpoint introduces a fair amount of lag, which is only compounded by the need to add an additional API call.
While I understand that a developer could cache the user id > username mapping, it’s an additional ask on top of an complicated transition where we’re already being asked to degrade (due to the new non-realtime nature of the new service) functionality that relies on this set of endpoints.