TD:
Continuance and customariness are two different lexical contexts
Continuance indicates an on-going action (e.g. Earth orbits the sun) whereas customariness indicates an action that is performed at intervals (e.g. I wash my hair).
That distinction is not relevant to the context of Burton's demonstration that both his examples (Luke 5:10 and 21:24) refer to things that start in the future, and then continue (or become customary).
At some point in the distant past (before the formation of Earth), an imaginary observer could employ the periphrastic progressive to say:
- Earth will orbit the sun (continuance)
- Jeffro will wash his hair (customariness)
- Jerusalem will be trampled (continuance)
- Simon, James and John will 'catch men' (but not in a gay way) (customariness)
In all these cases, it is a future event that continues in some manner.
Once the action has already started, saying it 'will happen' is no longer an instance of the periphrastic future progressive but the standard future progressive.