What would the Byzantine Empire have to do to survive?

[Robert Vermaat]

From the time of their reunification there was no point at which Justinian could have completed a reconquest.

Yes, I agree completely. The opportunity for expansion did not exist until perhaps Justinian's final years. It would have been a project for his immediate successors, IMO. But at least Justinian laid down the ground work for them.

That would have been the best way to survive - get access again to the western soldiers!
The western legions were always of better quality than the Syrian troops. Time and again we hear of the supposed higher quality of the gallic infantryman when compared to the Syrian troops. The posssibility of the Roman empire to move legions around was one of its big strengths.

Imagine the Justinian or his successor retaking Gaul! That would have been something...