You are the only one here who's positing that the only possible scenario involving Borg victories in VOY would have had to look something like this:
- Voyager flies up to a planet, Borg are attacking
- Voyager runs away
- Borg win
- Roll credits
You want to keep the Borg as effective villains (the way TNG overpowered them) this is pretty much the standard.
"Voyager doesn't stop the Borg or save the entire planet" does not have to mean "Voyager is completely ineffectual and ever last member of that species is assimilated."
If VOY is effective against the Borg, the Borg are weakened. If the Borg don't assimilate everyone present, then they're also weakened.
They weren't just spineless cowards, they were keeping priorities. Now, see if you can stretch that imagination and picture a similar scenario that could have happened in VOY, and you see our point.
VOY doesn't have any higher priority or missions other than "save our own skins and run back home". That's the premise of the show. Everything else is secondary, meaning there's no room for "We have a more important mission to accomplish, we can't stop to help." DS9 could tell that type of scenario because there was more at stake than their lives and they had actual missions to perform. VOY had none of that.
showing a defeat doesn't preclude an eventual happy ending.
A defeat that entails the utter decimation of an entire planet and its species, wherein the "heroes" were unable or unwilling to do anything but run away crying doesn't lend itself to anything but a bittersweet ending at best that still paints the "heroes" in a bad light for the rest of the series because of their failure/cowardice especially compared to the heroism of their peers/predecessors.
Kirk can fight Gods and save worlds from destruction at the hands of powerful invaders/destroyers.
Picard can defeat the Borg every major Invasion they send against the Federation.
Sisko can defeat the Dominion in their war for conquest of the Alpha/Beta Quadrants.
VOY's crew can...fail against the Borg and then run away, or just run away at the sight of the Borg despite their victims begging them for help the whole way through because if they manage to do anything significant the Borg will have been weakened as villains.