I don't see any reason not to have both original and fresh servers.
If they only did fresh servers, the population would be much, much higher than it was on the original servers, which was already too high for a lot of people. Even if they released a fresh PvP and a fresh PvE server, I think there would be population issues given how much publicity Nost has gotten over the past 6 months. Not to mention, only doing fresh servers would be devastating to a lot of people who don't want to lose their progress.
If they only do original servers, again, the population will be even higher than it was to begin with, etc. This time a lot of fresh players will be pissed because they have to start during the end of the BWL patch.
If they did both, then original players would have the option to play their characters or restart, and new players would have the option to start fresh or jump right into BWL if they so desire.
You can say the population would be split, but again, Nost has gotten an incredible amount of publicity, and since Blizzard pretty much said they weren't interested in legacy, I'm sure we will see a HUGE playerbase increase if Nost decides to come back. I don't think the fresh servers would be underpopulated at all, in fact I bet they'd be more populated than the original ones were. The original servers may or may not go down in pop, but I bet they would remain relatively stable. If the pop does decrease, I think a lot of people would see that as a blessing anyway, again because I think a lot of people agreed that (at least on the PvP server) pop was getting too high.