flybe don't need to instantly start flying all the routes, they can simply cherry-pick the ones they want and build up slowly.
I accept I don't speak with authority on this, but I've a feeling you aren't quite correct. I believe if a route lies dormant for a while (and I don't think it's very long) that route licence is forfeited, and a reapplication is required.
With the numbers of people leaving BACON, flybe may struggle to operate many of the BA regional routes come April. I accept, eliminating it's competition will still be a pretty useful development on it's own!