I am going to plagiarize myself, a bit pressed for time at the moment (bold italic are tiny addenda).
"I really don't want to sound dismissive, but I get the sense that most of you have only a vague idea about the history of Rashidun Caliphate and its successor states.
Best overview to read is God's Path: The Arab Conquests and the Creation of an Islamic Empire, by Robert Hoyland.
If you really are a fan of the Byzantines, then learning about the Rashidun does illuminate a lot of what happened in that crucial century when the empire lost 2/3's of its territory, and gives insight into what really could have happened. (I would add that not knowing in some depth about their other great enemies, the Mongols and the Seljuk Turks is a real handicap to understanding why history happened the way it did with the Byzantines.)
Just a couple of quick comments:
the Rashidun armies had already conquered Mesopotamia decisively through a series of battles culminating in the Battle of Firaz, where they heavily defeated a much larger coalition army of Byzantines, Persians and Christian Arabs. Persia was wrecked and was only waiting for the end after the Rashidun had dealt with the Byzantines. To use the old phrase: They were dead men walking, regardless.
No student of Byzantine history can let themselves not know about Khalid ibn Walid. Think Alexander the Great, Julius Caesar, Napoleon - yeah, he was on that level. Very unfortunately for the Byzantines and Persians. In fact, you want a creditable alternative to Arabic conquest of the later and near conquest of the former? Just erase him from history.
Finally, those Arab armies were highly sophisticated, well equipped, well trained, and ably led down through the ranks, and very devoted to a faith that gave them a coherent meaning and purpose, not a ragtag collection of desert horsemen who would melt away from setbacks, or even a true disaster. (What is it with underestimating both the Arabs and the emotive power of Islam?)
Mind, I am not saying that the Byzantines couldn't have done better against a new arch-enemy, but it could have been terribly worse.
Such is the riptides of history...
More of aside: People like to speak of more experienced troops as being superior, in fact, there is really a limit on how much experience you need to be a capable solider, trust me. There is also a downside, too much combat over the years can wear on troopers, and degrades their efficiency.
As for the numbers, everyone agrees that the Byzantines had a massive advantage. On the other hand, they were up against one of the greatest generals in history, Khalid ibn al-Walid. Arguably, as a force multiplier, he mostly canceled the Byzantine numerical advantage.
But all right, assume the best for the Byzantines on that battlefield, assuming utter destruction is pushing it, possible, but the Byzantines would pay heavily - and the Caliphate can quickly move tens of thousands of other warriors for yet another battle, and another. The Byzantines were quickly losing their numerical advantage as the Caliphate rapidly recruited - including some thousands of Byzantine deserters.
While it's clear that the Byzantines could have fought considerably better (that they didn't, in the face of what they knew was a very dangerous foe, is telling), even with the Emperor Iraklios in command I suspect the stars weren't in their favor on that battlefield that week. A lesser defeat would allow them a more delaying action; they might have held onto Antioch, for a while."