It is likely that if Gravity doesn't screw up RO2 'two' (because this is the second sequel they are making), and goes F2P with it, then it will completely wipe out RO1. I don't see this as a bad thing though if this comes to pass as it will mean having a lot of players, both former, not playing currently, or playing now - all in a bunch of public servers.
Now, if Gravity screws up, doesn't go F2P, or if something else happens - it probably won't wipe RO1. However there will be a drop if players in the RO servers as they try out RO2. In my opinion this is the worst situation that could develop because then Gravity will be stuck with 2 failing games.
Of course however, they may discontinue RO1 as soon as they launch RO2 to encourage people to play it. Should they screw up then that will be it for RO and it will be dead regardless of the name after it. Of course if they do it well then that will be the very best situation (no one playing old RO means they will have even more inclination to play new RO).