So this is an interesting question, if you unpack it. Phrased this way, you end up with a Monty Haul campaign, where the DM is nothing more than an enabler to the PCs. And, again, if that's what the table wants, then that's fine.
But not all tables want that. Various tables have various social compacts with the DM regarding "putting the fingers on the scales of justice," and/or "fudging," but your analysis would seem to indicate that the DM is supposed to always fudge in favor of player victory- because defeat (or a loss to a save or suck spell) would mean a lack of fun for that player, at that time.
But imagine you are a particular player that enjoys overcoming challenges. Let's say that, to use an example, you wanted to beat someone at chess. And then you beat him. Great! But if you learn that the player let you win ... not so great. The challenge was illusory. There was no thrill in victory, because the outcome was pre-ordained.
Same here. Some players (yes, even beer & pretzel & working 60 hours a week players) only enjoy victory as a counterpoint to defeat. Why bother planning out a battle and marshaling your resources correctly and, um, strategery if you know the DM will bail you out with some deus ex machina?
No DM wants players sitting and doing nothing for 40 minutes. But most players don't want their DMs cheating for them.
Just like no player wants to waste 10 hours of DM prep time, but most DMs won't try and railroad the players.
And, should a player be sidelined for a significant portion of the game (sent to Donjon or the Void, perhaps), then the DM should have alternatives available, including running the monsters in combat, running NPCs, or, if it is a high-fatality campaign, having each player have some backup PCs.