There are two ways to secure the blockchain with a guaranteed level of security. Proof-of-work, and proof-of-stake. I use the second term very broadly, and I'm not referring to the consensus schemes that NXT or BitShares use. Anybody who says otherwise today regardless of the "marketcap" of their coin doesn't know what they're talking about from a CS/game-theory point of view, or they're ignoring the costs of proof-of-work schemes.

Proof-of-work is expensive and will ultimately be replaced with better consensus schemes that require no mining, namely those that only rely on cryptographic signatures, a good balance of incentives and penalties, and an underlying byzantine consensus algorithm.

Disclaimer: I'm one of a handful of people working on new consensus algorithms.