what about the likes of nxt or soon to be nem? they are far superior platforms nem being more so. what makes then "unviable" in your opinion?
There is no known solution to the "nothing-at-stake" problem (also known as the "history-rewrite" problem) that all proof-of-stake (PoS) distributed consensus mechanisms suffer from. This has been debated ad nauseum since the original thread on July 11, 2011:
https://bitcointalk.org/index.php?topic=27787.0. Current PoS coins get around this problem, for example, by having the developers sign blocks as valid (Peercoin) or by introducing regular blockchain checkpoints into the code (Nxt). In other words, these PoS coins are not decentralized.
Even ignoring the nothing-at-stake problem, "dubious" is probably a better adjective than "superior" to describe these platforms.