Single agent boarding is a disaster. The one overwhelmed agent can’t manage boarding and last minute upgrades, clearing last minute standby passengers, and helping customers change seats – let alone answering questions from nervous flyers.
Perhaps even more importantly they can’t manage all the tasks that are part of departure while also scrutinizing carry on bag sizes (American is asking them to now charge to gate check oversized bags) and observe passengers who may already be drunk or under the influence of a substance which means that single agent boarding contributes to greater conflict on board the aircrat and potential flight diversions, too.