Why would 20 years later have been too late in your view?
Because when the Muslims attacked Syria and Egypt pretty much all the local Greek militiamen had been killed fighting the Persians, as had much of the Imperial legions. Twenty years later the next generation would be all grown up and prepared to defend the Empire just as they had against far grander threats than the Muslim invasion. Moreover, Heraclius' thema system (that was barely introduced when the Muslims actually did come along) would've been in full effect and halted the Muslims just as the more rudimentary system did in Anatolia.
Similar circumstances should apply for the Persian Sassanids as well.