This question already has an answer here:
Looking for a phrase/metaphor describing a situation where a proposed solution, though valid, is targeted for one of many problems in an entity plagued by so many problems as to render the individual solution unimportant. An example would be discussing improving the energy efficiency in the kitchen of a building that is condemned or on the verge of falling apart; though the methods for improving the kitchen might be totally valid, they're of little practical concern as they'd make no contribution to the long term viability of the building.
Something like "a drop in the bucket" but which emphasizes that the "drop" is well-conceived or -intentioned.
The answers here have been fantastic (and hilarious), though I think the phrase that best fits my request was actually listed by @user9383 in her/his question (not in the answer but in the question itself):
Fixing a leaky faucet in a burning building.
This phrase perfectly captures both aspects of my request:
The intentions/method have merit: in general, leaky faucets need to be fixed.
Despite this, the effort is wasted due much bigger problems: despite the inherent merit of the faucet-fixing, the ensuing destruction prevents the realization of any substantial benefit.