If every person in the game has both tanks at 50 and fully geared in i130 gear and can play a tank to perfection, but only 15-20% of people ever actually do this, there is a shortage of tanks. It is not enough to simply have the job levelled and geared. You have to be actually doing it everywhere a lot of the time, and as long as people are actively avoiding it there will be a real shortage of tanks regardless of the potential supply of tanks in the game.