There are solutions to this problem. It just starts costing a lot more money when you need to guarantee content.

That's because it isn't done legally, and therefore augmenting the network with a few "supernodes" (I believe that's what napster called them) is a big no-no. You could say the same about Netflix/HBO/..., by the way. Watching obscure things (that aren't worth Netflix or the copyright owner's time to negotiate rights for ... good luck). And if the present trend continues, and Disney, and Paramount, and ... start their own streaming platforms with exclusivity, well, popcorntime is going to be the vastly superior option in a year or two at most.

In the other case for a distributed example, you have the Steam platform. Obscure things download quickly and without much delay. And steam avoids the "rights negotiating" problem by being a marketplace.

That doesn't make p2p video streaming useless though. Or even inferior.

As if Netflix and HBO have everything you would ever want to watch?

That's not necessarily correct. Depending on the time of day and your ISP, P2P video can be much more reliable than Netflix. Esp because you usually control quality. With Netflix, it can happen that the client decides to force you on a low resolution because they think the connection is slow. That creates quality issues you don't get with P2P.