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.
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.
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.
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."