I don't understand the reasoning behind your conjecture but I don't see that happening because of the different modes of interaction. Messenger is more of a 1-1 channel that focuses on the immediate. Facebook is more of a broadcast channel focusing on passive, asynchronous, entertaining items.

WeChat has been successful with the model that you talk though .... Never say never ...

I don't see where Messenger could replace the Facebook native app at all...Messenger is growing every day and with bot support and SMS handling it's also becoming way more useful but from there to "Replacement of Facebook" there's quite the jump...

Most people use Facebook to communicate with their friends and family. With the regular Facebook platform everyone can see everything and your comment on a friends obnoxious photo might be posted on your moms news feed. Not so with Messenger. It's easy to have groups for friends, groups for family, groups for your Sunday football group etc. I bet most young people already are using Messenger a lot more than regular Facebook already.

Facebook is the last thing young people use.

I am not a young person but I am a graduate student with a lot of young friends. At my university, Facebook is primarily used as a carefully curated photo album and an event planner. The genuine social interactions on Facebook have moved into Messenger. Photos and videos that are not profession/parent friendly are only shared in group chats. Most people are in 3 to 10 active group chats.

A few years ago Facebook Groups were popular at my school but now only a few of the largest ones are active. When I transferred in to finish my undergrad, the groups were essential to meeting people. Now I think it would be much harder to meet like-minded students online because all of the genuine conversation is in semi-private group chats.

According to something I read, young people (<22yo) use twitter & snapchat for on-going daily updates and reserve facebook for bigger events like a family get-together.