Deprotagonisation means no longer being the protagonist, not simply not failing ever.
If I run a game and the PCs get their asses handed to them by the opposition, but that results from player choices and at all times the PCs remain what the story is about then there is no deprotagonisation, they just lost is all.
Deprotagonisation is when the story is no longer really about the PCs at all, but about something else (most commonly some GMPCs or metaplot NPCs).
Story here in the sense that any game creates a story of what happens in play, not some more abstract or jargony meaning.
So, if you're playing brooding vampires and the game focusses in play on your brooding vampires, you're protagonists. If in play it turns out that a bunch of NPCs from a sourcebook do all the interesting stuff while you watch, you've been deprotagonised. You may well have won every encounter, you're still deprotagonised though because ultimately what you did was not the most important part of the game.
And note, most important part of the game does not mean most important thing in the game world. If I run a game set in the Exalted universe about a bunch of mundane guardsmen then the PCs are not deprotagonised as long as the actual game we play is one in which what matters in play is what those guardsmen do. The size of the stage is not relevant, what is relevant is that in the game you actually play the PCs are the characters we want to hear about.
It's as simple as that. If you were reading this in a book, would the PCs be the main characters? If they are, they're the protagonists, if not they're deprotagonised. The fact that someone else in the same gameworld may be more powerful is neither here nor there. Rosencrantz and Guildenstern are the protagonists of Rosencrantz and Guildenstern are Dead, notwithstanding that Hamlet is a more powerful character offstage.