This point can be true to some extent, depending on how you estimate the readiness level of the overlord's own forces. It's possible that once Iran's "shield" of Jihadists are defeated, Iran itself has a much weaker personal army.
But this assumes Israel even can deplete Iran's shield.
There is a lot more Muslims than there are Ukrainians. And Russia sends a lot more firepower than Israel. Just because the "depleting the shield to expose the soft underbelly" might work in 1 case does not mean it will work in the other, especially because Israel has largely only managed to trade evenly with just Hamas so far, and has proven less adept at defending its economy and home regions than Russia.
The strategy you describe (run Iran out of islamists) works only if Israel can deliver sustained wins at rates enemies struggle to replenish.
The point is true in all situations. A force that you paid for and trained is always a valuable asset, not something free to be slaughtered at negligible cost. It has nothing to do with what happens after they are gone, whether or not the host can pick up the fight.
Has Israel only traded evenly? I highly doubt that. They are commiting genocide in a region that strongly supplies Hamas with its fighters. Killing large portions of that population indiscriminantly including women and children depletes Hamas' abilities to restock its forces and also clears the land for more Israeli settlements.
Although Israel cannot fully destroy its threats, reducing them, causing strain to them, diminishing their abilities to replenish the dead, and expanding Israeli territory, particularly while pushing out a hostile population right in the middle of Israel, are all victories and improvements to Israel's situation that are well worth fighting for. Even if Israel cannot achieve the full victory of clearing its surroundings of hostility so that it no longer has to substantially invest in defense, what it can do are all achievments and improvements for their nation nonetheless.
