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People will bandwagon and drop it eventually or take it to 50(60?) to keep their collection of max-levels complete.
Some might keep with it and some of those might be good. It's not going to be any different from War and Paly now.
There might be a few more tanks generated, but my money is that most DRK mains will be people that swapped from Paly and War.
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Remember that more jobs will be coming out alongside Dark Knight. People who are hopping on the Dark Knight bandwagon now may choose to go a different route once more info on what other jobs are being released too is given.
Dark Knights will be tanks after all, and honestly I feel as if them being tanks alone is going to cut down on the amount of people who are going to choose to play them beyond a certain point.
Better a tank where they'll be less... EVERYWHERE, than a DPS where they'll be the new Bard/Dragoon in terms of population.