I dunno, I kind of liked that Voyager went out of their way not to have everyone at lieutenant commander/commander like TOS/TNG did.
And honestly, if Voyager had bothered to do an epilogue instead of "oh they're home, the end" it's safe to assume Kim would've been promoted then, if from nothing other than sheer pity.
Honestly I think the real reason Kim stayed an Ensign was because Wang didn't endear himself to his co-workers.