Why was Seleucus able to hold down Persia after Alexander's death if the Macedonians were just foreign conquerors. Shouldn't Iran have crumbled beneath him and raised up an new Persian dynasty?
Well, the Macedonians were not the first foreigners to rule over other nation(s) / people(s) / countrie(s).
I wonder why it surprises you.
The Macedonians went against the huge multinational Empire ruled by the Persians.
The Persians had enough time to send several armies against the invaders - each time the Persians gathered all their available troops including their numerous warlike subjects of Non-Persian origin.
The Persians carefully chose the places for their battles, first they tried to get protection behind the rivers; then they tried an open field to make advantage of their (supposedly) superior cavalry.
And... the Persians lost every time.
It was pretty obvious to
everybody (including the Persians themselves) that the gods (the fate, the luck, whatever) left the Persians and sided with the Macedonians.
It was a fair game, both parties knew the rules good: if you lose - you lose.
There was no conception of "mandate of heaven" in this part of the Earth, but figuratively speaking the Persians lost the "mandate of heaven" and the Macedonians got it.
The Persians understood that they lost when they used the resources of the Empire against the Macedonians.
And now you want the Persians fight the Macedonians when these Macedonians have the imperial resources on their side?
It should be noted that the Macedonians in general (and the Seleucids in particular) were good empire-builders.
All the non-Persian peoples admitted that the Macedonians were no worse than the Persians, so there was no hope that the non-Persians would support the Persian resurrection against the Macedonian rule.
The Seleucids treated the Persians good, even used their bureaucratic managerial skills. But the Macedonians were smart enough not to allow too many former owners of the Empire into the Macedonian army (after the initial unsuccessful attempt of Alexander the Great, which actually failed after his death).
The core of the army stayed Macedonian/Greek. There were non-Macedonian (non-Greek) military detachments: Median cataphractes in the cavalry, the Jewish infantry garrisoning fortification, etc.
But the bulk stayed Macedonian/Greek, especially 'Grand army'.
And you hardly meet a lot of Persians in the imperial Seleucid army, as an exception maybe.
* As a side note - when the Parthians were taking the Empire from the Macedonians (which received a death wound from Rome), there was a resurrection of the local non-Macedonian population against the new Iranian pretenders in favor of the old masters, the Macedonians. This resurrection took place in Mesopotamia, but it was wholeheartedly supported by the Persia proper. Go figure...