You bring up some great points here, but you lost me at the end there.
I don't think there is any reason for a new detachment at this time (who knows about the future and future FO costumes). But we have several canon sources supporting the FN designation (Finn FN-2187, 2 credited FNs in the film FN-3181 & FN-8330, and the visual guide calling out FN-2003), at this point we don't even know if the TK mentions weren't mistakes (we don't approve white duct tape, but that's in the movies). Something not yet considered are the FO Flametrooper and FO Snowtrooper, where do those end up? The FN designation is the fastest easiest and most logical way to designate the FO costumes as being different and record them together.
As listed above (See #4) that designation can cover many costumes across several detachments. It could even just end up being the 501st designation for all FO costumes in Episode 7.
We have several 100% original costumes coming in from a new movie, that represent a new completely different evil organization, but we are just going to call them the same as those guys 30+ years ago? (But sandies, will be TDs, because ..... dirt.)