What are the biggest hurdles (technological or otherwise) that the Proof-of-Stake consensus algorithm must overcome to become the de facto consensus algorithm for blockchain? Or put differently, what's stopping PoS from becoming a reality, right at this moment?
And furthermore, in what ways is it so much more difficult to implement than a Proof-of-Work consensus algorithm? (I am assuming it is much more complicated by virtue of the years and years of R&D required to make it a reality, relative to PoW which was implemented 10 years ago now by one man, or perhaps a small team of people, depending what you believe)