Honestly, I hate people who "main" in general. Everyone starts somewhere, and I respect that. But I absolutely refuse to believe that any player who has put more than 100 hours into playing Overwatch cannot play at least 3 different dps heroes or 2 supports. I just don't see an excuse for not being able to and see it is as extremely unfair to act like you don't know better.
People always love to point out toxicity except for when they are causing it because either they A. do not care or B. are legitimately oblivious to the fact they are not doing enough because they have no measure or standard of what is sufficient in their role.
When it comes to the sexist bits of abuse and just flat out unnecessary hateful treatment of Widowmaker players, I will condemn that behavior all day long. But at the same time, many types of "main" players have drawn a lot of resentment to themselves and ugly as the divided community's response to them may be, they have brought it on themselves.
See, I say this because I can play so many different heroes and I am not afraid to try anything if I feel like it can make a difference. But I also have the competitive drive to win and am honest enough with myself to know when I am not satisfied with my performance and that something needs to change.
If more players were like me, I don't think there would be as much toxicity. Sounds like something anyone could say to scapegoat others for their lack of success, but it's just something I experience far too often to be completely sympathetic towards them. I would be more conflicted if they actually swapped when asked politely and were honest enough with themselves to know they are not moving at a pace that is not suitable, but you just can't win with them for two reasons: if you are nice, they expect you to become mean and if they swapped every time that someone asked, they would never get to play Widowmaker.
But the primary issue is this: they are playing Widowmaker to play Widowmaker. They are not playing to win with the confidence in their ability by adapting to circumstances and addressing the key issues their team is facing, they are playing just to shoot other players as Widowmaker and they would not want to participate in the match if they could not play Widowmaker. They are so stubborn that an enemy hero with vertical mobility ganking them and consistently shutting them down is more likely to convince them to swap than their own teammates are even if they are not being negative or abusive. On top of that, even if the team manages to inspire them to swap, by the time that they do it is already too late and they have wasted too much time.
I certainly do not condone the abusive behavior that they experience, but at the same time the fact of the matter is they are not taking the game seriously enough. This resentment was not formed in a void; this reuptation that they are remarkably stubborn, selfish, uncooperative, unreliable and possess an extremely poor comprehension of how to play the game was built over time.