Again, living happily ever after with the mother is fairy tale, but living happily ever after with Robin isn't?
That's just it though. The finale wasn't saying that Ted and Robin will
live happily ever after (if the writers wanted us to believe that, then they easily could have just shown
us that, like they showed us everyone else's future).
Instead it was only stating that these two-- who had something really special at one time-- will attempt to reconnect once more later in their lives. Maybe it will work, and maybe it won't. The point is that Ted decides (six years after the death of his wife) that it's at least worth it to try.
It's more of a hopeful
ending than a fairytale one.