I’m skeptical that this concern isn’t equally applicable to PoW. PoW is a form of money and it’s unclear how centralized/decentralized the current hashpower is. Certainly, the current mix is far to be optimal as a small project like #Qubic can wreak havoc the #Monero network of this sort. I think the biggest concern with PoS concentrating the coin supply has more to do with the supply held on exchanges. In this regards, #XMR delistings could be a blessing.
Proof-of-personhood chains (such as #Idena) is a mechanism that can prevent the centralization of the coin supply in PoS chain but that’s a niche feature for now.