Wagner Group captures headquarters of the Southern Military District and the building of the Ministry of Internal Affairs in Rostov, the headquarters of Russia's base of operations for War in Ukraine
How many do they have not in Ukraine... Who are also willing to kill fellow Russians? If the average Russian military sentiment is, "Eh, let them through" this could be huge.
Plus a lot of the "Royal Guard" may be well-connected rich kids who are even less inclined to fight.