I think we can agree that a majority of Nostalrius players that were present for the server's closure were not there for its launch. I remember how launch was utter mayhem, with most players not being able to connect at the designated launch time. When everyone was able to finally log in there were some 5000 people online which was UNHEARD of in the private server community.
After being very pessimistic about the future state of the old character realm and going on a bitch posting spree I have composed myself. In doing so I have considered FIVE reasons why the old server will still have a sizable population.
1. Server instability
Considering how many people are hyped for Nostalrius' relaunch and how many people want to start anew I predict that at MINIMUM some 7000 people will be trying to play at the same time on a fresh start server. I suspect the launch will not go smoothly as login issues and disconnects will occur. People will make new characters on the old character server if it is stable in comparison.
2. Starting zone madness
Most people will either make an undead character or a human and will not look the other way on their decision. These starting zones, just like on the day of the original launch, will be in chaos with players unable to complete quests in a timely fashion. This alone will cause players to make new characters on the old server.
3. Fatigue
After Nostalrius' closure many players rerolled Kronos 2 and leveled to 60. While some players will level all over again I predict there will be a population of players who simply do not wish to grind levels so soon and will return to their old characters. I also predict that due to the stresses of leveling in a densely populated environment where there is cutthroat competition for specific mobs and world items that players will become fatigued very quickly. They will come to the old server for a normalized leveling experience.
4. Lack of Alterac Valley
Alterac Valley is an important piece of content in this game. Not only does AV reputation offer important items to players, but it is an instrumental source of XP in the important leveling range that is the lower to mid 50s. Let us not forget that AV is entertaining and desirable herbs spawn there. Players will realize how important AV is and will come to the old server.
5. Attachment to characters and picking up where you left off
People have put 20, 30, 40, even as much as 100 days into some of their characters. Time is money, and leaving a character unused that has had significant time invested into it would be the equivalent of throwing away a considerable amount of cash. People, with their guilds, want to pick up where they left off. How many guilds were close to killing rag for the first time? How many guilds were finally starting to progress through Blackwing Lair getting important people geared for AQ? How many people enjoyed the guild members they surrounded themselves with on raid nights? The server closing did not mean people went their separate ways indefinitely. People will want to pick up where they left off and be with the people they care about again.