It sounds like even a little more mobility into the senate would have made a difference. Would the future barracks emperors have revolted if they were allowed a say in the next emperor, anyway? We’ll never know.

Well, probably they still would have. Most of them were already senators - some served as consul, even. Ambition is a hell of a drug. When professional militaries were being revived in Europe in the 17th century, a big concern (and justified one, considering the palace coups that would follow) was that the military might up and decide, like the Legions of old, that THEY could decide better who to lead the nation. And for a general at the height of his career, the soldiers acclaiming him is a very hard thing to turn down…

It’s actually a bit curious, the mixture of personal ambition and popular discontent. Generally, there was some amount of ambition involved on the usurper’s part, but at the same time, the Legions themselves were prone to spontaneous revolt when they were dissatisfied with the ruling regime - or just irritated at local circumstances! When a bunch of armed and suspiciously smiling soldiers wake you from your tent and tell you you’re Emperor, you have to be very careful how you respond…