IF the LISK killer/killers are still alive they will not be able to stop killing. Unless they've died, the best they could do would control it for a few years, or do just a single kill somewhere else to get their fix while laying low, i.e. B.T.K.
They'll eventually make a mistake as all eventually do, if they haven't already. Without DNA they'd only know it's him by confession, unless another victim has the same M.O. Like the burlap bags to connect them back to LISK murders.
If they're still alive, then they're not going to kill in GB again, or has moved somewhere else to start again. As high profile case this is, I don't believe they would stay around, or at the least the MO will definitely change.
Like many here have said, if he's still alive then only a very lucky unrelated arrest will catch them.
Personally I believe they live on LI, or in the vicinity where anyone from there wouldn't suspect them if they were seen parked along GB, allowing them to dump the bodies. Better yet it could be 2 people that both live on LI, and work together.
I'm sure it's been discussed, but has LE checked for the IP address of any people who contacted the Craigslist posts, or were they all contacted by phone? Even so there should be numbers on the phone records that point towards who it could be. Especially if the same numbers appear on several of the girls phones.
I'm a detective but these are JMO.