It was easier and faster to travel or communicate between Constantinople and Alexandria by sea than by land.
And that's how the grain shipments arrived.
The real vulnerability wouldn't be for an isolated Byzantine Egypt. It would be an for an Arab-controlled Levant, squeezed between a Byzantine Anatolia and a Byzantine Egypt.
The Monophysite restiveness was certainly an issue, but to date, it had not led to any notable revolt displacing Byzantine/Roman rule. It's too easy to read back native Christian docility to Arab conquest of Egypt as a resistance to Byzantine rule that was never really there before the conquest. They may not have been entirely happy with rule from Constantinople, but they hadn't led the province to revolt in any substantive way even during the Empire's most perilous days (the Persian Wars, etc.).
IMHO, this post wins the thread as by far the most sensible and reasonable account of how things were in Late Antique Egypt. Nice work, Athelstane.
At the
battle of Heliopolis, where 15k Arabs fought 20k Romans, someone other than Theodor leads the Romans. He inspires morale in his men, puts up propper scouts, interprets Amr's intentions correctly and defeats the Arab contingents in detail via sheer force of numbers before these have a chance to coordinate. (luckily killing off Amr during the opening stages might also help)
There, great man theory at its finest.
I'd be suspicious of those numbers. 20,000 men is described in the Strategikon as being an
exceptionally large Roman army of the period, and this is just a few years after Heraclius had basically been forced to rely on Turkish intervention because his own field armies were so badly mangled. I would guess that 20,000 men was not far off the combined total of the entire Anatolikon Theme (the Greek name for the army of the
Magister Militum per Oriens) in the 640s. And, don't forget, this is after the defeat at Yarmouk and loss of all Syria.
I would guess that Roman numbers defending Egypt were probably well under 10,000 men in total. In the circumstances, the Arabs could quite comfortably have had numerical superiority, or at least parity, which means a POD of a single battle might be somewhat difficult to do. A victory of any sort, though, will be helpful to the Roman cause. That said, the temporary reconquest of Alexandria in 645 didn't really do very much in the great scheme of things.