Here are the reasons why countering a tank is so much stronger than doing the same thing to a DPS or healer:
1. Tanks are the most interactive role
Tanks spend the most time interacting with the enemy team of any role. They are up front, with a big hitbox, charging at the enemy team. A lot of the time, tanks are taking on abilities from 5 other players at once (and some abilities from their 4 teammates).
So if the enemy goes zen, discord is unavoidable for your tank.
However, the Zen can limit how much they interact with the enemy by playing out of sight lines, playing further back, sitting behind tanks or DPS, etc.
2. Countering a tank means countering an entire role.
Imagine if one hero swap invalidated all the enemy’s healing and support abilities. It would be incredibly oppressive. That is essentially what is happening when a tank gets countered: your spacemaker can’t make space.
3. A lot of the abilities that would shore up the tanks’ weaknesses were on other tanks.
Discord/anti on rein->zarya bubble. Massive bastion/reaper damage on winston->defense matrix. Snipers eating up orisa->sigma shield and grasp.
These combos were ALSO oppressive. But now that they are gone, the pendulum has swung in the opposite direction.
4. Even though the tank line is weaker than ow1, tanks themselves are stronger.
In a vacuum, a tank is worth somewhere in between 1-2 players (not inclusive). So even though a Sombra pick is really bad into a 4 hitscan team, keeping enemy ball hacked is still well worth it.
Hope this helps people understand why people complain about tank counter swaps, even though counter swapping has always been a part of overwatch.