I note and agree with all of this. But it also seems to me at least worth mentioning, in the context of the question of whether the borders established by the 1949 armistice are "defensible", that Israel fought two wars from those borders. In 1956, the Israel Defence Forces seized the Sinai from Egypt in nine days. In 1967, Israel seized the Sinai, the West Bank, and the Golan Heights in six days.
There are certainly reasons why those conflicts might not provide good models for future conflicts. One might object that they didn't prove the borders were "defensible", since both began with surprise attacks by Israel against its neighbours. Perhaps one would need to consider the scenario of a surprise attack against Israel, such as the 1973 war, but fought within the 1967 borders. On the other hand, Israel's military superiority over Jordan and Syria is now much greater than it was in 1956 or 1967, particularly since the loss of Syria's Soviet patron. In any case, it just seems like people ought to at least make some acknowledgment when talking about the defensibility of these borders that these two wars did actually take place, and the borders didn't seem to be much of a problem for Israel.