Discord will not replace Reddit. It's a chat client. I don't understand why people keep saying it's a replacement. It's not. I have tried!
I know it is incredibly inefficient with the same questions being asked and answered constantly, but it appears to be what we do now. But even with that downside, it creates more intimacy/connection instead between members. So it likely leads to more engagement, even if it is inefficient.
But that has not stopped some servers from exploding in size. And its "fine" if you are just trying to shoot the breeze or shout into the void, not look for specific information (which is truly horrendous) or answer FAQ (which is even more horrendous than reddit).