Well you kind of right about the way dps are combination of both roles in the damage and size way, But every class has their reason to exist, tanks can do damage but because of their huge buddies it’s easier to kill them…
Support have small size and supporting their team (Heal or shields [Symmetra]) with low damage.
The role of the dps in the game is to counter both classes and deal damage, Even though tanks can deal damage their damage is slower and except for Orisa more based around close range and they’re more based around shielding and protecting with huge health pool and shields.
the dps job is to take out key targets and pushing in(offense)/repelling the enemies (Defenders). Try doing a match with 3 tanks 3 support or 4 tanks 2 support and you"ll see that Even though the have a lot of health they can not deal with 2-2-2 because they do not have enough damage to break the shields or a way to get around it without behind cought…
I hope this post helped 