The answer is always more nuanced than yes or no.
It’s always been the case that a chat box, shoutbox, Discord etc, will take attention and traffic from the forum. The rapid response cycle alone somewhat guarantees a divide of attention.
However, this is not to say one will “kill” the other. They can, under the right circumstances, be a suitable complement - it is normal now in the RP scene for example, to do plotting and planning and the general community “off topic” stuff lives on the Discord while the actual storytelling lives on the forum. But this has its own perils and foibles for sure.
As long as there is still a healthy amount of discussion of longer form (that never suits a Discord/chat feature), the two can co-exist. But the forum will inevitably be quieter in the off-topic areas for it.