That's because high rates can only go so far. Well, compared to low rate.
Mir is dying because of high rates? Ever thought about not playing 'em? If your gonna be annoyed everytime a server goes down and you have to level again, why not do some research before playing... how long it's been online, user count, etc etc.
You see, I think your opinion is wrong. Mir is simply dying out because it can't or hasn't been developed enough. Just think for a second, if Mir was to be released nowdays it wouldn't stand a chance. It's not because there's a new 1 every other day, it's not because it's repetative, it's purely because the gameplay isn't even near a level worthy of making it big in the charts today.