Given all of Harry’s duties and responsibilities on Voyager. He doesn’t need to be an ensign to do that job. Plus, with Harry being considered a senior staff member. He definitely didn’t need to be an ensign.
Especially when you consider that Voyager had an operations lieutenant on board (Durst). Harry should have been subordinate to him, just as academy dropout B'Elanna should have been subordinate to Carey. Ironically, these actions would have done far more for Harry and B'Elanna than Carey and Durst... Harry would still have been elevated to chief of operations upon Durst's death, but he could have been played as a Starfleet kid who wasn't ready to command a division (but no one else was qualified). B'Elanna wouldn't have effectively gone from "violent hothead" to "valued team member" in one episode, squandering her potential growth as a character.
Harry being an ENS initially makes sense as he was more equivalent of Uhura (the ranking bridge officer in Operations and Communications, but subordinate to a higher ranking officer within the same otherall division (Tuvok versus Scotty) and on a much smaller crew. Also, a promotion to LT-JG wouldn't actually make any difference as he already had his own quarters from the outset, which is the only tangible advantage that we know of.
If rank is not an issue, then why was it a punishment for Tom Paris to be reduced to ensign, and an honor for him to be put back? Also, why did he get a lieutenant's pip in "Unimatrix Zero" for a year and a half of exemplary service, when Harry was stuck at ensign after six?
Hoshi is a more complex case as honestly she was done far more "dirty" with her being an ENS in the first place, but IMO if you accept that and that she had no change in position (bear in mind, unlike Kim JG doesn't appear to be available), then her not being promoted makes sense.
Hoshi (and Travis, who was in the same boat and was probably the most competent member of that crew, probably due to a lifetime spent in space) can be explained if you figure two things: first, the Starfleet Officers corps had four non-flag ranks: ensign, lieutenant, commander, and captain. The "half-pip" ranks of LTJG and LCDR don't exist. Second, the show only covers four years: Riker's holographic version wasn't accurate. Could a modern recreation of a Naval action that happened in 1815 have every detail correct?
It's still a stretch, though. Especially for Travis, who as far as we know never made a mistake. It's easy to see why the "Dead Stop" station wanted
his brain for its collection... it was the best one on the ship.