The reality of short sentences is a major problem. But the intelligence services have learned a lot. They follow them pretty closely. Once they get out, they are five and 10 years older. They might have other projects, be less violent, have other opportunities. They might fall in love. Some have less energy, they cannot do what they could do when they were 20 or 25.
Europeans often criticize U.S. counter-terrorism methods, but you see the American approach to religion as a kind of buffer against extremism.
In the United States, in spite of 9/11, it is a society which understands religion much better than in Europe. Muslims can be practicing and devout without being treated as if they were fundamentalists. Europe is clearly different from the U.S. Islam is the religion of the oppressed in Europe. Most Muslims are working class. There is an underclass that is comparable to the black or Latino underclass in U.S. cities.
The major threat in Europe is small groups that are difficult to spot. Like the two Lebanese who planted suitcase bombs on a German train in 2006. Jihadists are fascinated by 9/11. They want to do something cosmic, apocalyptic, but the intelligence services are all over them. They cannot succeed. The small groups, of less than five, are the most dangerous.