There may also be use cases for private blockchains, and there are startups pursuing those, such as http://chain.com (though I'm not sure why regular databases wouldn't work for most of those use cases), but I think there is also a need for a global decentralised network (or networks) for other use cases - global transactions/contracts, decentralised companies, probably other use cases yet to emerge.
Current scaling/latency issues with blockchains - there is ongoing work to solve those via off-chain networks, sharding etc - it's still early days, it will take some time to develop.
Data storage is also a moderately interesting application. Decentralizing data persistence, and being able to take advantage of the enormous amount of excess hard drive space that exists in the world is an interesting concept. Things in this category are Sia, Storj, and MaidSafe. While i'm not sure the economics and logistics will actually work out, I do think there is something there.