My guess would be that they were headed in that direction.
My speculation here has simply been that this'll be what triggers the (inevitable) breakup.
I grew up in the 60's, and was a young adult during the 70's. Those were crazy times, I saw plenty of couples take this route.
I never saw any of them NOT eventually split up. (Although sometimes for -nominally at least- other reasons.)
I suppose there are exceptions to this. Rules are made by the exceptions. If there are any, however, I never met them.
I think my bet is about as safe a bet as I could make. Time will tell.