But if we assume that the sonic fence was built to keep Smokey out, then it has been around since Dharma got there (and built the fence). Also Richard said to Horace when he walked into the compound (earlier this season) that the fence may keep "other things out" but not him. I am sure he was implying the Smoke Monster when he said that.
So I think the smoke monster has been roaming the island for a long time and well before the incident.
And, not that it's been said or anything, but I think it is important to remember that "The Incident" isn't the end all major event that it sometimes feels like it is being made out to be. Life at Dharma does go back to normal afterwords. We've seen life on the Island and with Dharma years after the incident. Ben is a workman, and so is his dad, they still deliver beer and supplies to all the stations, etc... and there seems to be some sort of truce with the Hostiles for a while, at least until the purge. So whatever the fallout of the Incident and the events leading up to it, and with Ben being shot and all the other crazyness going on this season life does go back to "normal" for Dharma for at least a few years (Ben has to age a lot over that time) before the purge happens, the Swan has to be built (and a few other stations), etc... SO the Island can't go completely to hell after the nuke goes off... unless the future can be changed... In which case anything can happen.