Should I stop playing mages this season?

Because A is only superior to B if there is a B...to pretend that Tanks do everything better than Mages would be foolish. Mages still bring stuff to the table that Tanks cant

Mages tend to lead to more smooth sailing/functional team comps

reliable AOE damage, poke, wave clear, late game high reliability 100/75-0 burst damage, cc, range etc, they generally give you late game scaling superiority

Assassins create a liability in team comp that has to be covered by superior mechanics and/or the whole team picking around the assassin pick

An ap carry will always be valuable for damage diversity, though at the moment that's basically the only reason to have one.