I see it lasting anywhere between 150 - 350 more years, because it was quite well run. Any more is pushing it.
Yeah. A lot depends on how much is dragged down with the dynasty. To use the ERE, dynastic collapse meant little to the state's survival. Someone would take the throne and the system would keep running.
But while well-run, the Achaemenid was loosely tied together. A collapse of central authority will see satraps take advantage of a chance to be kings of their own kingdoms.
That's my half-educated guesstimate.
And Egypt would DEFINITELY break off at some point. That may even be what hastens the decline/collapse. The Egyptians were among the most rebellious of the Achaemenid's subjects.
Yeah. How this works out in the very long term (1000+ years) is anyone's guess.
If memory serves, ImmortalImpi said at least a good part of that was Greeks stirring up trouble there - but that they had people who would be stirred indicates a troublesome province.
And across Syria and Mesopotamia is an uncomfortable distance to reach across to break heads. Doable, but awkward.
Not something that will last in days of imperial weakness.
And anyone's guess on Central Asia. That area seems likely to spawn trouble at some point, for one reason or another. But that's another sort of problem than Egypt.