Thanks for the references Ankrom..
What you've provided is one theory of how Iranians view the conquests. I don't wanna assume a 'Bias' in it although I've seen the tendency in many Iranian sources to devalue the Arab conquest and attribute it (as much as possible) to external reasons without giving sufficient credit to Arab effort....
As far as I know, the Parthian Empire lasted until around 230 AD or something, and since that time the Sassanids were in control. So this makes their reign until Arab conquests close to 400 years....It is debatable that any Parthian influence have survived that long.
On another note, the Parthian history itself is not comprehensive, at least to the degree of the Sassanid....Below is an abstract from Wiki: Native and external sources
" Local and foreign written accounts, as well as non-textual artifacts have been used to reconstruct Parthian history.
Although the Parthian court maintained records, the Parthians had no formal study of history
; the earliest universal history
of Iran, the Khwaday-Namag
, was not compiled until the reign of the last Sassanid ruler Yazdegerd III
(r. 632–651 AD).
on Parthian history remain scarce, with fewer of them available than for any other period of Iranian history.
Most contemporary written records on Parthia contain Greek as well as Parthian and Aramaic inscriptions. "
Now looking at the issue again....
There is no denial in any source that the Persian state was weakened, for sure that was one reason for the conquest. However, the point is that weakness is a political one, one that had no material bearing on the quality of the Persian military force.....
The proof is obvious, the whole campaign took several years to fully overthrow the Sassanids, it had above 10 or 11 battles (other than seiges), in all battles fought the Sassanids outnumberd Arabs by at least 3:1 or 2:1....
It makes sense to have the Sassanid weakness as a supplementary reason but in no way that could qualify as a 'main' or 'core' reason for the conquest !!....Credit should be given where it should be, Empires don't fall so easily....