Earlier quoted context omitted.
It holds up if you assume war crimes are beneficial to your goals but there is quite a lot of evidence, and sophisticated theory going back to clausewitz, that they mostly aren't. They can look useful at a certain level of conflict, but once you are thinking of war as being a tool for accomplishing policy goals (how modern nationstates view it), a lot of the things you would "want" to do stop being useful. Wars that…
Using human shields and hostages worked. Hamas still exists because of it. Dark times ahead.
That's normally where the international system has an influence, through sanctions or simply refusal to support the conflict, or deciding to support the other side, etc. Intentionally killing civilians would almost always fall in this category, but israel has apparently unlimited will to do it and is effectively unsanctionable in the current political environment, so it will continue.
Anyway there are much more illustrative examples that prove the rule, for example landmines. They aren't currently considered war crimes generally, but they are extremely damaging to civilian populations during & long after the conflict, and most countries have signed the treaties banning them. The countries that never signed are exactly the ones plausibly expecting to fight a war soon: US, china, russia, israel, iran, india, pakistan. And now some eastern european countries have withdrawn as well for similar reasons.
So from that you can kind of infer that landmines are probably very effective at their military goals, in a way that eg summary execution of prisoners or bombing hospitals may not be.