I've noticed people that spend most of their time in group chats/Discord like communities don't tend to use Instagram/Snapchat/Facebook that often and vice versa as well. I think that's because "spider-web" (I am making these terms up on the spot) style platforms allow each individual person in the community web to feel like the center of their own world, whereas the more "roundtable" -esque discussion programs like group chats/Discord decentralize the individual and facilitate longer form conversation and community centric interaction. It's much harder to feel like the center of attention in like a group chat vs. on Instagram/Snapchat with stories/posts/likes/etc. Vice versa, its harder to feel a part of a consistent, reliable community entirely on Insta/Snap/etc since interaction tends to be fleeting and surface level.
So, I don't think Snapchat will directly eat Messenger's user base because much of its UI and functionalities are focused on that spider-webby, contract inwards towards the individual style social media, which Messenger does not have. It's the same idea as Facebook separating itself from Messenger; Snapchat's UI has too much noise in it taking away from Messenger's appeal.