"DOS is still great, Why does it need a replacement?"
If something is robust and efficient, why does there need to be something new to replace it? It serves the purpose and solves the problem well, why would you replace it with something new simply for the same of something new?
DOS isn't a great example, DOS is a much bigger system than something like BT. BT is more of a protocol than a piece of software, and even in that case there are piece of software which have existed for decades without the need to be replaced for the reasons above.
New does not mean better, and just because something is old does not mean it's bad or there is something better out there. Perhaps we come up with something that comes after torrents, but we shouldn't do it for the sake of it, it should be because torrents don't work for a use case or we've come up with a faster / more robust way of doing it.
It works perfectly fine as it is in my opinion.