Piketty explicitly warned the middle class that they were an endangered species, a fragile thing that erupted to life in the wake of the French Revolution, had a brief boost of life after WWII but are historically an anomaly, a discontinuity in the general social order of very, very few rentier and an undifferentiated sea of nasty/brutish/short serfdom. Up until 1861, the Russians could buy and sell 90% of their population.
I thought there was a hint or two of "Lock your mansions, the masses could potentially mobilize if you guys aren't prudent" kind of thing?
No, it's much more depressing than that. It's an economics book, not a politics book and its argument is basically "the only time the poor and middle class claw back any money from the rich is during times of great social upheaval and war." It basically argues that if you aren't busy getting rich, you're getting poor.