It would be a blockchain-based neural network which people can sell training data to (in exchange for the underlying cryptocurrency) and which other people can use via an API to do pattern recognition on their own user data (by paying in the same cryptocurrency).
It would be a decentralized, autonomous AI.
1) It costs more and isn't as performant as tuned machines. 2) The training data is very valuable and shouldn't be shared on a blockchain and restricted to a closed network.
Maybe there is one advantage though. Crypto people have bought up all the GPU's.
2) People get paid to provide quality training data... So yeah it's valuable but if there is a built-in financial incentive to share data with the system then people will do it. Also, other people could get paid to verify/curate the training data. The system just needs to setup the right incentives.