51% mechanism is what allows to challenge the status quo, thus enabling competition for power.
Just sitting on your money in PoW does't give you control over the system, just engaging in the competition for control doesn't bring you money as mining is normally a break-even game. Thus money and control become orthogonal, which results in a dynamic system without asymptotes. In PoS you get both in a nice single package, which saturates fairly quickly.

Hybrid systems are more tricky beasts, thus they need to be studied closely. I'm concerned that saturation in PoS aspect of the hybrid will at some point begin to negatively influence the competition on PoW side.