I'd like to start by saying that I don't believe Discord is a huge problem for RPs. GMs can decide for themselves if they want to use discord or not. 

But I am not a huge fan of discord. I like the slower pace of forum OOC posting, as it's much easier to keep up with. While I'm at work, I can miss several important conversations on discord. That never seems to happen when everything is kept on the forum. That, and it's just an extra place where I have to check for questions that were directed at me. And since I like reading forum posts, the fact that the OOC banter has almost entirely vanished is a negative to having an RP discord.

I just find it kind of distracting. It's really a matter of preference, and I don't think you're going to have much luck debating people on it. You're comparing apples to oranges.