Here is what I think will happen:
1. WoW Classic is released.
2. Players OVERWHELM the WoW Classic server. There is far MORE interest in WoW Classic than they anticipated. The initial Classic playerbase is something like 65% of the retail playerbase.
3. The zones are congested and packed. It becomes difficult to level simply because you wind up with a queue to kill certain quest mobs.
4. The people who vault ahead of the pack are those who ignore pretty much all quests from 1-25 and just grind. They hit 25+ ahead of everyone else and then get places like Arathi Highlands or Thousand Needles to themselves, which means they can now quest and pull ahead even FASTER.
5. Blizzard decides to take no action regarding congested zones, adopting a policy that "the people wanted a vanilla experience and this is part of it."
6. The Classic playerbase drops as people quit because they won't tolerate congested early zones. However, this is balanced by some people quitting WoW altogether, even retail. So the playerbase drops to something like 50% of retail.
7. After a couple months, WoW Classic begins to settle in. Players adopt a policy of switching to Classic every time there is a content drought in retail.
8. Every time there is a retail content drought, people play Classic and realize that, for some reason, its just more FUN and INTERESTING. They have a reputation in the Classic community to defend and protect. They MATTER. The gear and accomplishments seem to MATTER in a way it does not on retail.
9. With every content drought in retail, Classic actually converts people to exclusive Classic players.
10. 6 months after launch, the Classic population is about 75% the size of the retail population.
11. 9 months to a year after launch the Classic population is roughly the same size as retail.
12. People begin to regularly clear Naxxramas by now, with the PvE content complete, people quit WoW Classic. But thee people also feel retail is somehow inferior and do not return to retail.
13. The WoW Classic population hovers around 80%-90% the size of the retail population.
14. At this point, with the PvE content complete, the WoW Classic server begins a permanent transition into a PvP server. WoW classic becomes exclusively the place to grind rank 14, or play Arathi Basin, WSG, or especially old Alterac Valley all day.