Your argument belies reality, where moderators even on the very large subreddits turn over all of the time.
The oldest moderator on r/news has only been in their position for slightly more than 3 years: https://www.reddit.com/r/news/about/moderators
The oldest moderator on r/news has only been in their position for slightly more than 3 years: https://www.reddit.com/r/news/about/moderators