I was talking to someone recently who works as an advisor to G4S and he was saying that the French are still alarmingly underprepared. There are a lot of potential shooters out there and they are really struggling to keep a lid on it.
As you say, the number of potential shooters in France is staggering, for a lot of reasons. It's impossible to monitor them on an individual basis, nor is it to put them in preventive jail (although some argue that the French government took a very worrying step in this direction by using home assignment, most of them having been judged illegal by courts months after they began). The French DCRI has limited funds, although they have steadily increased in the past few years.
Anyway, political terrorism is not solved by putting 10 millions of people in jails. El Baghdadi became who he is now in jail. Terrorism is fought politically, either by negociations when they are possible, which is not the case here, or by drying out the embodiment of what these guy fight for, namely ISIS for Islamic terrorism, the USSR and the communist ideal for Italy in the 70s-80s etc.