Apparently mounting a 51% attack on a blockchain can sometimes net more money than the compute cost! https://blog.bitquery.io/attacker-stole-807k-etc-in-ethereum-classic-51-attack
miniblog.
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Filecoin is solving a real consumer problem (needing backup storage but having empty local capacity,) using blockchain protocols!
https://filecoin.io/blog/filecoin-proof-system/
Blockchain problems can sometimes be replaced with simpler tech, but this seems like a great match.
On adoption of blockchain technology and finding compelling use cases:
https://www.mckinsey.com/industries/financial-services/our-insights/blockchains-occam-problem