If you get FA you can not get detailed into another branch. FA is a recipient branch, not a donor branch - you can get detailed into FA from MI, SC, or other donor branches. You can not get detailed out of FA.
Popularity of branches changes over time. Once MI was middle of the road in popularity, now it is red hot. EN is still popular, but nearly as much as in previous decades.
As far as popularity among athletes, I think likes attract likes. Once a few members of a group (such as a football team) go in one direction, others tend to follow - the Supe is a football player and FA. Also, FA is an excellent branch; it is a combat arm with good promotion potential.
