To be fair, the same youths in Germany or the UK that are technically-not-unemployed aren't faring much better, working below minimum wage or ridiculously precarious jobs (like the infamous 1€/hr jobs in Germany or the 0-hour contracts in the UK).
I'd argue this "French economy" argument is pretty weak under scrutiny.
General disillusionment is a characteristic of the entire European south (or EE) in general. I don't think it has more to do with islamic radicalization than other kinds of radicalization though.
That makes you want to get a job or raise unemployment subsidies, not go kill people in the street..
With 25% youth unemployment, wanting to get a job doesn't mean getting a job. People give up after a while, and the resulting desperation is dangerous.
Makes different people do different things in response. Idle hands, etc. Throw in unwelcoming locals, radical preachers in the community, stories of oppressive foreign governments on your home turf, etc and I can see how idle youths find a call to arms appealing.
The French economy is moribund, growth is nil, youth unemployment rates are over 25%, and lots of them are sitting around unemployed or in dead-end jobs - while their parents hold much cushier jobs with some of the world's best labor protections (or most-notorious, depending on your perspective). It's not shocking.