i'd say the primary motivators should be to roleplay a character and to craft a story for yourself and others to play in and enjoy
more seriously though rank turnover here has never really been an issue I've seen. people leave or go on break and surrender their rank or screw up royally and get bumped or killed often enough that progression still feels natural (to me) tbh. i've never been in a position, in fleet or MI, where i've felt like my character was in a position where s/he was not where they needed to be and was entitled to a position. if your character is good for a spot, then i see that more often than not the character will get it, despite what the roster currently looks like