There's usually between 60-90k online in both BO1 and MW3 whenever i go back to play them, and you can still find lobbies in MW2, WaW and CoD4 (although these games no longer have that much players and they all have pretty severe issues with hacks), so there's probably going to be plenty of players in BO2 for the next 2-3 years.
The games don't use actual servers, they work with player hosted matches and use listen servers for that, there's probably nothing to monitor as far as hacks goes. The older games were also built with less security and at least the devs have said they would fix them if they could (although the newer ones are already hacked too, and have been for some time now) and they don't have have a in game reporting system so there's less chances of getting caught.
And any company will eventually stop customer support for an old product that no longer produces revenue, unfortunately with this series that usually happens when the next game comes out.
Won't even be buying Ghost, this company has gotten it's last dollar from me
Most likely, most players will move over. You will be left with people who can't afford to get Ghost, people who got BO2 from a friend or 3rd world country kids. The only reason I would even consider going back is that if Ghost had problems. On top of that, people will be moving on to the new PS4 and will probably sell their outdated PS3.