Some of this is chalked up to the impregnability of the Theodosian Walls (Theodosius d. 395 AD), only ultimately overcome by siege cannon in 1453.
(The fall to the 4th crusade is often attributed to treachery and infiltration rather than overpowering conquest).
The preservation of the control of the capital always seemed to leave a base from which important hinterlands could be recovered.
So for the 800 year period, 400-1200, how plausible is the destruction of the the Byzantine Empire?
A) Not plausible at all
B) Unlikely in the extreme, but barely plausible
C) Medium plausibility
D) Probable enough that it is surprising the empire survived those 800 years