Again few words about mongols.
svn 468/600ad/normal/Italy.
The year 1972, mongols not only didn't collapse, but they are even solid. even after i killed about 30-40 of their units with tanks and nuked them...
Also, as I told earlier, mongols just became weak. In this time they didn't conquered China, Russia or Korea or someone else before 1500. In my games they never doing it after they were nerfed.
But they did it later. In 1700s conquered China except 1 city and Korea except 1 city. Vassalized both of them. After it they became really strong and solid.
And i'm pretty sure, that the reason is that they have a lot of cities in a core area. They are close to each other and almost useless, but anyway they are in a core.