Most have another hero they can swap to in case their pick gets taken, such as in the video, it was stated that Fuey500, the Torb One Trick, would occasionally play Reinhardt. Chro can play Mei, Kephrii can play McCree etc. etc.
I can’t think of a situation where you NEED a Zenyatta/Moira. You can easily play around it, Mercy is a pretty versitile hero. It’s equally selfish for you to yell and cry in voice chat should they not swap to a hero they obviously will have little experience on, forcing them to play “Your way or the highway”. You don’t HAVE to babysit them, you don’t need a shield for the Bastion One Trick, odds are that they know how to play without a shield if they’re a one trick.
Avoided, sure if you don’t like playing with them, but banned is extreme. They have a right to play the game just as you do. I don’t care if “it’s the way the game was meant to be played”, if it was they would put it clear as day in the report options that only playing one hero is worthy of a ban.
You cannot have the rules only apply to one part of the game and not to the rest. It’s all or nothing. Either it’s ok or it isn’t.
And you want to know the best part about all of this? There already is an answer to whether One Tricking is ok or not. Here you go: