It is actually not that toxic. It's just that a lot of people que up for activities that are way beyond their capabilites and then get declined. And then complain about how toxic the game is. But infact, it is the other way around. Those people are the toxic ones. Not the ones that decline them.
For example, someone who hasn't cleared EP Heroic at all shouldn't que up for a EP Heroic speed clear, right? It is completely normal that those people get declined. Instead, they should que up for something appropriate. Like a EP heroic progress run. The same goes for Mythic+. If you have never done a level 10 key intime, then why do you que up for a level 15 key. Do the activities that are appropriate to your gear, your class and your experience. Another example is to que up for a level 24 key, as Warlock or Priest. Those classes are just bad for it. It already is a level 24 key and will be very difficult in itself. Why hinder your group even more by taking bad classes?
Now given... EP or a level 15 key doesn't require specific classes. Any class can do activities like that. But the mindset of players is still the same. Some classes are better than others, making the key way easier. Less stress, more fun. As simple as that.
Lastly, all the people that do activities that are extremely difficult usually don't go to MMO-Champion and complain about it. The people that complain are the ones that get declined. As such, it naturally creates a toxic atmosphere. What those people should do, instead of complaining, is to actually play the game. Climb up the ladder. If you want to do high keys (whatever high is for you), then invest the time for it. If 15 is your goal, you should atleast have done a 13, maybe a 14, for that specific dungeon. The same goes for raids, the same goes for pvp and even for real life. Nobody is going to hire you as a comic artist, for example, when you have literally 0 experience. Just stop expecting to get free shit in WoW only because you pay for it. That is not how it works, sorry.