Actually the opposite will happen. Every time the price suddenly increases some supernodes cash out and take profit. If the number of supernodes decreases too much they will lower the required balance to fund one. This increases distribution of XEM and thus increases decentralization.

The more the price increases, the more early adopters will cash out increasing the spread of distribution. The higher the price goes, the harder it is to accumulate as well.

I also think that PoS is superior to PoW because to participate in the 'shared-trust' of the network, you have to buy in from an existing stakeholder unlike in PoW where you only need to acquire mining hardware. It would be incredibly difficult to secure enough XEM to successfully the attack the network. In PoW, it would be very hard to detect a bad actor who is accumulating or researching advances in mining hardware to attack the network.