In most games, Tanks and Supports have capabilities that the rest of the roster do not have/do not have to the same degree at the cost of the capability in another area.
Generally, the other area is damage but, occasionally it’s something else such as movement.
If the Tanks and Supports don’t have to pay a significant cost you can easily end up in the position where choosing primarily or exclusively Tanks/Supports can be better than picking DPS.
If you look at my hero selection history you’ll see an example of this:
When I first started playing Overwatch, I played both DPS and Supports nowadays, I play supports most of the time and I play Overwatch less. The reality is, on a support I can get far more done than on a DPS and the gameplay of supports isn’t good enough to keep my full interest (Some of that is due to support changes made by the dev team).
The thing is the design behind DPS heroes tends to be more enjoyable for many people that the design behind tanks and supports. Displacing the DPS line up effectively limits the size of the games appeal.
2-2-2 may mandate that DPS are in each match but, that doesn’t mean they get much of anything done while in the match. This is especially true for players with average or bad aim.
The situation is made worse in part due to the DPS heroes predominantly being glass cannons and the maps generally being about control of relatively small areas. If the tanks can actually get into range to engage the DPS it isn’t pretty for the DPS even when the tanks have lowish damage and as the relevant areas on the maps are quite small the DPS is frequently in the tanks range.
It’s hard to say.
Roadhog is massively annoying to fight against which can create bias against him (I’m personally biased against the character). At the same point, he’s spent most of the game’s history costing his team matches by feeding the enemy team ults which slaughter his team. Last I knew he was having average to above average success but, a tank that combines with him well is currently very dominant so it’s hard to say how much of that is him with the public stats.