Disclaimer: Sorry for using the terms "good" and "bad". It's just the simplest way to do it.
Raiding: The Legendary system made good players go back to earlier tiers and even do LFR to farm Legendaries. I remember that even when TOS was out I still did EN to farm off-spec Legendaries. This allowed bad players to complete content much more effortlessly and progress in their own way.
M+: I'm not sure if people remember this since it's many years ago now, but when Blizzard presented the M+ system, it was actually designed to make good players help bad players. The bad players would make their keys available for the good players to get AP and in return, the good players would help the bad players complete content. Back then you were not able to reset keys as you can now, and failing a key would result in it being actually depleted. So good players were forced to use the keys of bad players. Blizzard actively expressed that this was partly the intend of the system. Of course it didn't work out quite as they wanted it. Good players mostly ended up spamming Maw of Souls keys. But this problem was mostly due to the fact that longer dungeons didn't give more AP which is a simple design flaw and could easily have been fixed. They should simply have made dungeons like HoV and Arcway give more gear/AP than dungeons like Maw of Souls from the beginning of the expansion.
I consider myself a "good player". And to be fair I'm not going to help bad players unless there is something in it for me. "Community solidarity" is not enough. Right now the only incentive to help bad players is boosting. And we see how this ruins the game. The LFG is full of "WTS" groups. And it also ruins PVP.
Personally I wouldn't mind helping bad players, as long as there is some kind of incentive in it for me. And I think unless Blizzard make some in-game reward structure to incentivize helping bad players (like they did in Legion), we wont see the boosting problem being reduced.