The third war showed that the Imperial Empire can take on a waagh and win. While a massive leveraging of troops took place, it was nothing compared to what met the 13th black crusade.
Ghazgkull will attempt again, and get beat back again. However, if he was to capture Armageddon on his third attempt (remember Angron is accountable for one of the Armageddon wars) then the High Lords would react in kind. His waaagh would be smash under the hammer of billions of guard and multiple marine chapters.
Besides, it's not the waaagh itself that is dangerous. It is the spores left behind, that harass worlds long after the main fighting has ended.
CJ: Coherent thoughts would be great... Did you fall asleep at the keyboard and just type "WAAA" instead of finishing the word with a "GH"?
