This seems kinda' stupid. Sometimes people joke around by saying "You're so dumb" or "Haha, you failed," if it gets changed to something positive, then that's super awkward and annoying.
That's the point. To discourage players from using toxic language / say toxic things. If it's going to be awkward or annoying, then hey!, good job! You probably discouraged a lot of people to use those words/phrasese now.
Plus, this will be really hard to implement.
While i do believe it's impossible to come up with an opposite for everything (like
@MegaCoin said), i don't think it's 'really hard to implement', it would rather be time expensive if you were to go with the route of trying to come up with an opposite for every single scenario (which i don't recommend). I personally wouldn't mind seeing an opposite for the classic toxic phrases like: 'F*** You, 'You suck', 'You're a noob', 'easy', 'noob' (as it's own word), 'mother**ck*r'. Stuff along those lines. I wouldn't mind and actually like seeing atleast those phrases to be given an opposite, not everything has to have one, but atleast very 'popular' toxic words/phrases.