Well, if the qanats are so vital to the region, wouldn't that limit the points an invader actually needs to control?
If the Assyrians can indeed conquer the region (probably using an oversea route rather than across the mountains, I'd think, at least for Fars), then they'd control all the water, thus making rebellion difficult.
That's a good idea, but there's a caveat. For one, most non-Persian empires ruling Iran failed to realize the importance of the qanats or fund them properly--you see this with the Seleucid Empire, Abbasid Caliphate, etc. who let the infrastructure of rural Persia go into somewhat of a decay.
Secondly, the Assyrians' greatest enemy was not their non-Akkadian/non-Aramaic subjects. Rather, it was the king's own vassals and even his second and third sons who became his worst enemies. The second son of Esarhaddon,
Shamash-shum-ukin, caused a devastating revolt in Babylon that almost destroyed the Assyrian Empire 40 years early. Meanwhile, it was not an infrequent occurrence for an Assyrian city to go through multiple rebellious governors in one year, each one rebelling, getting killed, and being replaced with a purportedly more loyal governor, who rises up and is defeated, ad infinitum.
If the king of Assyria conquers the Persians, he will have to place a viceroy there. At some point this viceroy will rebel and now he controls the water supply and the mountain passes. Defeating this rebellion, which will be just as hard as conquering the area from scratch, will allow other constantly simmering rebellions in Mesopotamia and the Levant time to grow--possibly fatal for the empire.
The Assyrian empire was basically a giant experiment on how large of an empire can be maintained purely by constant, brutal military campaigning and juggling one revolt after another.
I don't think Egypt itself was much of a threat to Assyria proper; were Canaan and Egypt a net gain for the Assyrian Empire, or would the resources they spent controlling them be better used combatting their greatest threat, in the East?
Well Egypt was only briefly under Assyrian control and later became a puppet. Actually, Pharaoh Necho II was the Assyrian Empire's first true ally, attempting to save it during the Babylonian invasion. Egypt is definitely better as a puppet state or independent kingdom.
Canaan was rich and close to Mesopotamia, so controlling it was definitely a net gain for the Assyrians.