There are couple of things the OP doesn't take into account. First of all, blockchain and cryptocurrencies are different. Blockchain is the technology while cryptocurrencies are products build on top of blockchain technology.

Blockchain is a great decentralized system that uses many computers resources and there is no central decision making. I believe this technology has bright future ahead of it.

There's a reason cryptocurrencies aren't widely adopted. Currently around 6-7M people in the whole world use crypto currencies to purchase goods or services. That's too little market adoption so price can be stable. Therefor, currencies value fluctuates every single day which makes them quite risky. Transaction time is also very slow. Bitcoin can process around 5-6 transactions per second, Etherium - around 20 and Visa - 1700. Imagine the whole world start using bitcoin tomorrow. Everything will collapse in a day.