I think them using Athena as the match announcer NPC was a clear indication the Overwatch team decided to have her be that instead of a playable hero. It's not like the Junkertown Queen where they can edit one map to remove her voicelines if they feel the need to, Athena is already 'busy' as the match narrator and that'd be a much bigger change to the entire game. And yeah she COULD keep being the announcer plus a playable hero, but that would be confusing especially for new players.

Also... what's so great about Athena? She's a robot voice that talks to Winston sometimes. What has she ever done or said that makes her desirable as a hero, compared to a new hero like Echo who can be what you expected Athena to be? I think people just got so used to putting 'Athena' at the top of their hero prediction lists that they convinced themselves she 'deserves' it or whatever.