A dungeon can be cleared without a tank. Tanks are useless and content needs to be made to make them more necessary.

A dungeon can be cleared without DSP. DPS are useless and content needs to be made to make them more necessary.

A dungeon can be cleared without a healer. Healers are useless and content needs to be made to make them more necessary.

https://www.youtube.com/watch?v=f4O4JGCC0g4