Yes, exactly. It's not a case of "rich get richer," so much as it's a case of "capital is continuously and irreversibly re-invested." In equilibrium, mining breaks even.
so over time, the rich miners get richer at a faster rate than poor miners.
whereas under PoS, everyone gets the same % return (unless you stake with a pool, but even then you don't have these economies of scale like with mining.)