I think, looking at Nordeus financial and game player stats figures, that it doesn't really matter about players past level 10 or so.
They realise people quit after around a year and so they are concentrating on new players all the time, which they seem to be doing quite successfully, with the South American and Asian market in the last few months being particularly lucrative. In fact, very successful.
This is how it'll go, the game probably has around another 24 months of high growth rate (plenty more million $$$ yet) then they'll move on and develop another money spinner.