The police have done a pretty good job on cracking down on jihadist organizations (who aren't exactly covert), but occasionally a lone nutjob will still slip through.
The likelihood of a big, organized attack, such as the one in Paris, has grown much smaller, but these sorts of indiscriminate acts of violence can hardly be prevented, not at this point anyway.
With ISIS pretty much done, the morale of jihadists to commit themselves to attacks is very low. But as usual they are pretty much just waiting for the next big terrorist group to rise. These sorts of "sleeper cells" still do exist, and need to be purged, before the Middle East gives rise to another big jihad organization to fill the vacuum.