This is not true and has never been true unless the Widow is smurfing.
Widow does rely on her team, she is not completely independent from them. Widow relies heavily on sightlines, and she cannot consistently get good sightlines without the assistance of her team making space to where she can more safely move into a better position and get a better angle.
No, she cannot because without being able to reliably secure kills, she will not be a good hero, nor will she be very enjoyable to play.
This is a lot more than simple “mindsets”. It’s customer satisfaction. People get more satisfaction from seeing a noticeable impact from their actions and individual choices than they do when they watch others get that impact.
There might be some outliers and anecdotes, but I am willing to bet that the vast majority of players prefer when they aren’t “on the sidelines” so to say.
Also, you mention specifically you play tanks and supports and not dps and “carries” as if implying tanks and supports don’t have a noticeable visible impact. To this, I remind people that damage and eliminations aren’t the only way to see the impact.
To use your example, as Ana, you can sleep “stun” a nano’d Genji and you will feel far more satisfied than you would have if you had not, or if sleep just made the Genji have Z’s over his head. That is because you can see the impact you had right there very clearly.
Yes, I know, and solo killing does not mean no team play.
That is the issue with Widow and Hanzo to a lesser extent. Though you describe it as “2HKO” which isn’t at all very representable in the game. The issue that Hanzo and Widow have is in part to do with their fire rate. Fire rate is a lot more important than theoretical “2HKO” since the 2 hits don’t account for time in between those two hits.
Hanzo and Widow might very well do too much burst damage, but the way to fix it is a lot more complicated than just reducing damage and giving them a CD to compensate.