I agree with you that probably FB will stay longer than everyone thinks. At least the messaging part. What I think it's the biggest factor that will give it inertia, it's the userbase.

It has slowly become the centralized place for connecting with people, especially when not considering the minority of very tech-savy people.

I am at a party and I met somebody I want to hang out with? He/she's probably on facebook then we'll communicate on that. Wanna look up for my old class-mates? Let's search for them on facebook.

The timeline will be empty/boring/spam, but the messaging app will be full.