I remember showing a friend this really crappy online video stream in the late 90s, and I think he said something like "How can this ever be useful? I can rent a DVD and watch high quality video without any interruptions." He wasn't wrong.

That's just simply not true. Stock markets / futures markets are not decentralized in any sense of the word. You still require a 3rd party if you're going to trade derivatives over the counter.

Tell us how you can trade stock derivatives without a intermediary like a broker.

I agree that this particular example may not seem useful (since there are other, centralized ways to trade AAPL derivatives), but the useful thing here is that it's all within the same ecosystem. I've traded for years using brokerages, clearing houses, and banks, and this removes all of that overhead. The pegs are maintained by arbitrage and this allows the removal of almost all of the overhead associated with futures markets. But that's really just the tip of the iceberg. I'm not here to convince anyone; you should convince yourselves that it's either useful or not. There's certainly a lot of real development going on if you know where to look, and the hype is warranted if you can see the whole picture.

24/7 open trade market that can't be closed or shut down, no middleman fees (just network fees which will trend lower as scaling solutions are brought online).