Did you really mean "minion"? If she truly increased your value, that isn't really the best description of her ("
a follower or underling of a powerful person, especially a servile or unimportant one.")
I've been in a similar situation - sometimes you just grow tired of the job and feel that you are no longer "learning" or feeling that you have a chance of career advancement, and like your PA, having more money offered to me wasn't the reason I wanted to leave. I wanted to be challenged, increase my knowledge and yes, advance my career as well. Sometimes it's not about the $$$ value, it's about the feeling of being valued.