The long story short is that you are being made to (by default) give up rights that you should have, particularly around class action lawsuits. It's strictly bad for you and strictly good for the company. They probably shouldn't be allowed to do this. Since they are, the only thing we can do to protest it is to opt-out.
Maybe you'll never sue discord. But maybe someday there will be a lawsuit brought against discord by someone else. A few ideas for topics might include a security vulnerability that leaks personal information, the use of discord content for AI training data (e.g. copyright issues), or the safety of minors online. If you don't opt-out, you can't be a part of such lawsuits if they ever become relevant. This overall weakens these lawsuits and empowers companies like discord to do more shady things with less fear of repercussions.
And, since the vast majority of people will never opt-out (since you're opted in by default) these kinds of lawsuits are weakened from the start. That's why every company in the US is doing this forced arbitration thing. At this point, they would be crazy not to since it's such a good thing for them and the average person doesn't care enough about it.