The only primary source we have on the topic we're discussing? Damn right you should respect his account.
First of all, use the civil language. Then, bring yourself up to date with a military history.
And it probably escaped your attention that military history has evolved somewhat since the late 19th century;
It evolved even more since the time of Herodotus.
Delbruck's writings came out of a period where 'professional expertise' could outweigh the written primary sources, which today few historians would attempt outside of exceptional circumstances.
Which probably is supposed to mean that they would repeat any nonsense as long as it was written by the ancient author (BTW, Herodotus is hardly a "primary source"). But it looks like they are basing the estimates based upon the same approach as Delbruck, aka, 'professional experience' (size of the camp, etc.).
Moreover, if no modern historian has tackled the issue of Persian armor, which our only source says they had, denying it is just arbitrary and baseless skepticism.
Well, there are numerous depictions and archaeological discoveries but, anyway, you clearly don't understand what was written:
existence of something like scale armor does not automatically mean that everybody could afford it.
Anyway,
"Геродот: “Потерпели же персы поражение главным образом потому, что у них не было тяжелого вооружения и они должны были сражаться легковооруженными против гоплитов.” Наличие некоторого количества возможных льняных или чешуйчатых доспехов без хорошего щита персов не спасало."
http://strategwar.ru/military-history/drevnee-vooruzhenie-persidskaya-armiya
Translation: 'Herodotus: "The Persians had been defeated mostly because they did not have a heavy armor and they were forced to fight lightly armed against the hoplites".
Availability of some numbers of quilted and scaled armor without a good shield was not saving the Persians. '
Modern historians estimate the Persian army at Plataea at a bit under 100,000...
Then, you should be happy with what Delbruck wrote: based on the distance the Persians marched in a day when Athens was attacked, he concluded that 75,000 was the upper limit for the size of the Persian army, including the supply personnel and other non-combatants. In his battle account of Plataea, Delbrück estimated the Persian army, including allied Greeks, as amounting to 40,000. But, anyway, even 100K is a huge step down from a sacred "primary source" which as you insisted most of the modern historians are using: according to that "source" Persian army was over 300,000.