It's on the tip of my tongue.
Example: "Replacing the hard drive of this computer would be [idiom]. It's going to fail completely soon enough."
"Grasping at straws" is probably the best I can think of but I think it is a weak phrase.
Consider 'futile' or '... an exercise in futility.'
as @Joseph-Neathawk wrote.
or just simply "doomed" (which is just an adjective not an idiom)
From a different perspective
"Replacing the hard drive of this computer would be pointless. It's going to fail completely soon enough"
In the computer trade the idiom for this is polishing a turd:-
you can't polish a turd
1.(vulgar) Something inherently bad cannot be improved. [Wiktionary]
although I think this reference slightly misses the point, which is, that you can polish a turd, but after you have finished polishing it, it remains a turd.
One of interest that sprang to my mind:
"Like polishing brass on the Titanic."
Which I gather was popularized by author Chuck Palahniuk in Fight Club, but exemplifies what might normally be a worthwhile act overshadowed by the big picture.
Short and sweet and it's an idiom: not worth while. When doing something that will not save time nor money in the long run, we can say
Replacing the hard drive is not worth while
Related is the following 17th century phrase, not worth the candle and its longer version the game's not worth the candle. Meaning that the task at hand does not even merit the cost of lighting a candle.
Replacing the hard drive is not worth the candle. It's going to fail completely soon enough.
closing/shutting the stable door after the horse has bolted
trying to stop something bad happening when it has already happened and the situation cannot be changed Improving security after a major theft would seem to be a bit like closing the stable door after the horse has bolted.
Actually not an idiom, but a simile. There must be other expressions as well.
"fight a losing battle" may work here.
to try hard to do something when there is no chance that you will succeed (usually in continuous tenses)
Example (from above link):
We try our best to cope with the workload but we're fighting a losing battle.
What about flogging a dead horse? Not the usual context for the idiom, but it seems to fit well enough.
the juice isn't worth the squeeze.
"Too little, too late."
"A day late and a dollar short."
"Like spitting in the wind."
"A wasted effort."
"Like putting lipstick on a pig"
The first that springs to mind is:
You can't bail faster than the leaks
This relates directly, because it deals with the save-ability of the thing, rather than more-abstract futility. "spitting into the wind" is about futility, or worthlessness. Saving / salvation, is another matter.
I think I've also heard an expression like
"alms for thieves"
... or similar, but I don't recall where – so I'm not sure that it can be as reasonably applicable. The 'thief' here is 'not possible to save' with charity, because they are at odds with the principal dynamics of the situation. That may not always be true to the measure of whether somethings is "beyond saving."
If you want to say "beyond saving", then say just that. It's a perfectly good way to express your meaning. As others have noted, your missing blank can perhaps be filled with pointless or futile.
My first thought was "a fruitless attempt"
Example: "Replacing the hard drive of this computer would be fruitless. It's going to fail soon anyway."
as useless as a fence around a cemetary.
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