Why are people worried about players moving? Because they see their own view playing their favourite MMO and maybe switching with their friends. What they seem to forget is how many MMO's are out there. We see our western created MMO's as the prime of the market, but to be honest they aren't. Measured on population there are MMO's in Asia which are far bigger than World of Warcaft. If you count non fantasy MMO's too the population of Warcraft seem to be small compared against the playerbase of for example Neopets Online.
There are many fantasy MMO's free to play out there too. And every week I am stumbling over more. Many of them are localised versions of Korean MMO's, some aren't. Still there are so many free massive online games out there that you might wonder why they didn't hurt the population of WoW, WAR or AoC.
The answer is simple: target audience. Casual Gamers vs. Hardcore vs. "Hey I am new here, what is a MMO?". I heard once a statement from Blizzard that many of their WoW gamers claim that WoW is their first game they have played. Thats a big statement and worth considering why WoW attracts those people. Some hints: WoW runs on old machines, its very easy to use and explains everything you need to play it. At level 1 you need 3 buttons and a mouse, thats all. Compare that to the complexity in WAR or AoC.
Anyway, my summary is simple: no new MMO launch will hurt the player base of an existing MMO much. If a playerbase is diminishing look for other reasons.