PoS is decentralized as soon as you have a big number of stakers. The hard thing being “how do you know that there are in fact a big number of people and not just a bunch of guys hidden behind a huge number of aliases” (the Sybil attack problem) and this is for this exact problem that proof of work helps (contrary to popular belief, Proof of work has nothing to do with double spending).

For this reason you cannot build a trust-less distributed network with proof of stake alone: you'll need some kind of proof of waste to bootstrap it.