Smerel, you say: “Chazal say that when Rav Yochonan Ben Zakai saw that those who were supposed to fighting in defense were cooking straw in boiling water for nourishment he said “could they possibly win over the soldiers of Titus ?” He therefore went to meet Vespasian. It is pretty clear that he was not opposed to their fighting per se . It was he realized it was a lost cause any way he tried to get the best deal possible.”
Let me preface by saying that I agree that the Chachomim has nothing against fighting off dangerous neighbors, and there are Mishnayos to that extent.
But in this case, the full story as the Gemara relates is that the Chachamim said at the outset that they should not fight the Romans, because they won’t succeed. However, they were not in full control and the Baryonim did what they did. Then, they burned the storehouses in order to force everyone to fight. The Medrash quotes Rebbe Yochanan ben Zakai as complaining about this action.
It was after this, obviously, that Rebbe Yochanan ben Zakai said that someone must be done because there is no hope at all to continue.
That while episode was before the actual Galus, there’s not much to glean in this regard, anyhow. And obviously the inhibition to fight was not because of any Shvuos.